Pull up the carpet in the rear passenger footwell. Open the plastic tray with the wire bundles. Locate the wire bundle going forward from the door lock pump. Inside that bundle are two lumps you can see and feel, which are crimped connections for the door lock power and ground.

Typically the crimped power connection gets corroded and goes bad, which causes the system to stop working or become intermittent. Remove the tape, cut out the old crimp and renew it. I stripped the wire insulation back and soldered the wires together.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC