Position sensor external to transmission. Had same issue with daughter's 97 Escort until mechanic found it.
There could be a few reasons on why your 1977 Mercury Mountaineer is not going in reverse. The most common thing that could be wrong with it would be the gears are broke.
clutches.
Reverse and forward gears
There are many reasons why your reverse gears stopped working. There could be a leak or a plug for example.
6 gears not including reverse
Well in most cars there are 5 gears and the reverse. Don't forget the timing gears, steering gears and differential gears.
nope no reverse just 6 gears fast stuff
no oil in the tranny will cause the bands to streatch and will only allow your trany to work in revers
If this is an automatic transmission, more than likely the transmission is on it's way to the junkyard. Usually failing transmissions lose reverse first, then the higher forward gears - last will be the lowest forward gears.
Reverse is usually the first to go, followed by first gear. Time for a transmission rebuild.
get a new clutch
There are 3 foreward gears and 1 reverse gear.