Metra 70-1786 Radio Wiring Harness User Manual

Product Overview

The Metra 70-1786 Radio Wiring Harness is designed for seamless integration of an aftermarket radio into select 1999 to 2004 Land Rover Discovery and 1991 to 1999 Mercedes Benz vehicles. This harness plugs directly behind the factory radio, allowing for the retention of the original factory amplifier. It features 4 RCA plugs that connect to the pre-amp outputs of your aftermarket radio, eliminating the need for cutting factory wiring.

Image 1: The Metra 70-1786 Radio Wiring Harness, showing the main vehicle connector and the four RCA plugs for audio output.

Image 2: Packaging for the Metra 70-1786 Radio Wiring Harness, displaying product information and installation features.

Setup and Installation

Before beginning installation, ensure your vehicle's battery is disconnected to prevent electrical shorts. This harness is designed for a plug-and-play experience with compatible vehicles, but some aftermarket radios or specific vehicle configurations may require minor adjustments.

General Installation Steps:

1. Preparation: Carefully remove the factory radio from your vehicle's dashboard.
2. Harness Connection: Connect the Metra 70-1786 harness to the factory wiring harness in your vehicle. Ensure a secure fit.
3. Aftermarket Radio Connection: Connect the RCA plugs from the Metra harness to the corresponding pre-amp outputs (typically labeled Front Left, Front Right, Rear Left, Rear Right) on your aftermarket radio.
4. Power and Ignition Wires: The harness provides connections for constant 12-volt power and accessory/ignition power. In some Mercedes and Land Rover applications, the 12-volt constant wire (Yellow) and accessory wire (Red) may be reversed. Consult your aftermarket radio's wiring diagram and your vehicle's specific wiring to ensure correct connection. You may need to swap these wires if the radio does not power on or retain settings.
5. Amplifier Turn-On: Ensure the amplifier turn-on wire (often Blue or Blue/White) from your aftermarket radio is connected to the corresponding wire on the Metra harness to activate the factory amplifier.
6. Testing: Before fully reassembling the dashboard, reconnect the vehicle battery and test the aftermarket radio's functionality, including power, sound, balance, and fade.
7. Final Assembly: Once confirmed operational, securely mount the aftermarket radio and reassemble the dashboard components.

Important Considerations:

• RCA Plug Compatibility: While this harness features RCA plugs for pre-amp outputs, some aftermarket radios may require direct speaker wire connections. In such cases, the RCA plugs on the Metra harness may need to be carefully cut and the wires spliced directly to the aftermarket radio's speaker wires. Ensure correct polarity (positive to positive, negative to negative).
• Factory Amplifier Retention: This harness is designed to retain the factory amplifier. Vehicles with complex factory audio systems (e.g., Bose systems) are generally compatible, provided they use analog signals and not fiber optic connections for the amplifier. Newer Mercedes models (e.g., W163 from 2001 onwards) may use fiber optics, which are not compatible with this harness.
• Vehicle-Specific Features: Be aware that some vehicle-specific features, such as steering wheel controls or third-row seat audio (e.g., in some Land Rover Discovery models), may not be retained or supported by this harness. Additional interfaces may be required for these functions.
• Speaker Configuration: In some Mercedes models (e.g., SLK 230), the factory setup might only have two front speakers with rear speakers acting as low-frequency units, and the amplifier manages frequency allocation. Ensure your aftermarket radio settings are adjusted accordingly (e.g., set to FRONT or ALL, not just REAR if no dedicated rear speakers are present).

Operating Instructions

Once the Metra 70-1786 wiring harness is correctly installed, the operation of your aftermarket radio will be as per its manufacturer's instructions. The harness facilitates the electrical connection between your new radio and the vehicle's existing wiring, including power, ground, and audio signals to the factory amplifier.

• Power On/Off: The radio should power on and off with the vehicle's ignition, or as configured during installation (e.g., constant power).
• Audio Output: Audio will be routed through the factory amplifier and speakers. Adjust volume, balance, and fade settings on your aftermarket radio.
• Settings Retention: If the constant power wire is correctly connected, your radio's settings, presets, and clock should be retained when the vehicle is turned off.

Maintenance

The Metra 70-1786 Radio Wiring Harness is a passive component and requires minimal maintenance. Once installed correctly, it is designed for long-term reliability.

• Periodic Checks: If you experience intermittent audio or power issues, it is advisable to check the connections of the harness to both the vehicle and the aftermarket radio to ensure they remain secure.
• Environmental Factors: Ensure the harness and its connections are protected from moisture and excessive heat within the dashboard cavity.

Troubleshooting

If you encounter issues after installing the Metra 70-1786 harness, refer to the following common problems and solutions:

Specifications

Warranty and Support

Warranty Information:

The Metra 70-1786 Radio Wiring Harness comes with a 1-Year Limited Warranty from the date of purchase. This warranty covers defects in materials and workmanship under normal use. Please retain your proof of purchase for warranty claims.

Customer Support:

For technical assistance, installation questions, or warranty inquiries, please contact Metra Electronics customer support:

• Helpline: (800) 253-TECH
• Website: www.metraonline.com

When contacting support, please have your product model number (70-1786) and purchase information ready.