Poor man's Scanguage (or SuperMID)
 

For those of us who have an upshift light but don't have a Scangauge or SuperMID (I'm on Yoshi's list for a SuperMID whenever he restarts production), I wonder how difficult it would be to bypass the switch that disables the upshift light when you are in 5th gear, so it would function in all gears. The upshift light monitors vacuum pressure and comes on when it is in the range for economical driving, and goes off when you are at higher throttle settings or when your accelerator is backed off far enough to be engine braking. This would be a simple mod to do (assuming you could get access to the switch) and hopefully there aren't any other engine functions that use the output of the 5th gear switch.

Do they use the reverse switch? It might make sense if 5th and R are on the same side of the 'H' pattern as do most Japanese products, and both gears would have no place to upshift.

It they used the reverse switch as a dual purpose switch, what keeps the backup lights from coming on whenever you are in 5th?

OK, I could be wrong ;)

Why not use a vaccum gauge?

the "switch" that controlls the light is most likely built into the PCM.(i know for a fact that it is on the s-series GM picklups) no real way to modify it unless you had HP tuners software where you could change all the settings.(but its expensive)

the reverse switch has two steps in it, one for reverse, one for 5th gear, the switch with both should have an extra wire to wire in to the upshift light controle box.