Unplug the engine harness and the rear light harness from C500. Then plug in only the rear light harness, Key on do the rear lights come one? If so then you know it is the wiring in the engine harness that is causing the lights to be on.
On some MT Fiero harnesses there is a plug for the reverse light when you have a 5 speed transmission. The plug is located on the branch that runs past the alternator, to the VSS and then to the reverse light switch. If these wires are shorted together (and not to ground) then your engine harness will cause the reverse lights to come on. - So IF IF the reverse lights come on when the engine harness is plugged in (key on) but don't when the engine harness is unplugged (key on) then starting at the VSS start unsheathing the engine harness till you come across the Lt. Green and Dk. Blue wires and then tape them so neither connects to anything.
The connector for the reverse switch will have a Dk Blue and Lt Green wire to it.
The two wires need to be not connected together and not connected to anything else.
|Edit: I just had another look at the C500 diagram above and there are 3 wires for backup lights: C1, E1, and F9. Two are light green and one dark Blue. Does the second light green on the engine harness side of the C500 come from the transaxle switch?|
C1 - Dk Blue - Key on power from the fuse box. Runs to the engine harness and that connects to the reverse switch.
E1 - Lt. Green. - Runs to the engine harness - Gets connected to the Dk. Blue when the 5 speed transmission is in reverse.
F9 - Lt. Green - Runs to the reverse lights. Rear light harness. (Not the engine harness)
Under C500 you will find the Lt. Green from your console reverse switch connects to E1, which also has a jumper that connects to F9. So electrically all the Lt. Green wires are connected together when both plugs are in C500.
See the above diagram if you are confused about the Lt. Green wires.
[This message has been edited by phonedawgz (edited 09-12-2014).]