Request a Mercury Car Radio Stereo Wiring Diagram Whether your an expert Mercury mobile electronics installer, Mercury fanatic, or a novice Mercury enthusiast with an Mercury, a Mercury car stereo wiring diagram can save yourself a lot of time. Automotive wiring in Mercury vehicles are becoming increasing more difficult to identify due to the installation of more advanced factory oem electronics. Mercury Grand Marquis Stereo Wiring Harness - AutoZone Order Mercury Grand Marquis Stereo Wiring Harness online today. Free Same Day Store Pickup. Check out free battery charging and engine diagnostic testing while you are in store.