SUPER simple. I told a buddy of mine to do this to his 98 that's rockin' 01-06 tails. Looks great...and he painted 3157's red with normal paint and it lit the housing up really well.
1. Cut the signal hot and cut the backup hot, splice the signal hot into where the backup hot used to be. and cap off the backup hot, as it'll still be hot.
2. Trim either your signal socket/bulb holder so that it'll fit into the reverse hole of the housings. OR trim the housing so that the signal/bulb holder will fit in the reverse spot on the housing. That way...no crimping, no hot wires to accidently ground out. Just trim that lamp. it's a no biggie. Dremel FTW!
'01 XLT disguised as a '03 Fx4 Level II
Big. Loud. Slow. Bright.