If you don't solve it with the above suggestion, you can use a digital multimeter to figure out which wire it is by testing for resistance - you would attach one lead to the wire that goes to the sender, then test wires under the dash until you find the one with a resistance close to zero. Presumably you would only have a couple wires to check. Just make sure the end of the wire at the sender is not attached to ground or anything else when you're testing the wires under the dash.
1989 Bravura - repowered with 2001 PCM 351 GT-40 - 1.23:1 PCM Trans.
1989 comp - sold
1985 comp - sold