backup lights, neutral safety switch problem

[72freak]

When I built my 1972 Suburban I used the running gear and wiring harness from a 1990 model Suburban. The transition went flawalesly (almost). One of the few problems I have yet to work out is the backup lights. I believe on the existing 72 harness and neutral safety switch.....the backup lights only get power when the truck is put in reverse. Well, on the 90 harness and switch it works the other way...the back up lights are hot all the time and the switch completes the circuit. (I think) Atleast this is how they are working at the moment. Example....when it is all plugged up......the reverse lights stay on all the time and when I put the truck in reverse....the lights go out. I have swapped the wire on the plug going to the switch and that didnt work, I looked into switching the neutral safety switch thing to the 90 model one but that wont work. I have also looked into taking the 72 switch appart and re-working it on the inside and ofcouse there is always re-wiring the entire back up curcuit.

Have any of you had this problem? Is ther an easy fix?