I'm not sure what you're asking. The backup lights don't come through the
dash (firewall). Roughly near the top center of the dash, (looking under
the hood) you should have a square plug with four wires which leads to the
transmission. This goes to the backup light switch and the park/neutral
switch if you have an automatic. Manuals have the same plug but have a
jumper wire on the connection to close the loop. The other connection is in
the left kick panel under the instrument panel. where the rear lamp harness
connects to the dash harness.
And no, the backup lights are a dedicated circuit. No other lamps come on
with them.
> Does anybody know what two wires out of the fire wall are for the back
> up lights for my 70 mustang fastback? The previous owner decided to
[quoted text clipped - 3 lines]
> source. Are the side marker lights supposed to come on when the back up
> lights are on?