A very likely problem is the "Stop Cap" (020 301 266) on the selector shaft
is broken and flopping around.  This is the bearing for the left end of the
selector shaft and is the specific part of the selector shaft on Bentley page
35.60a that the "Selector Shaft" pointer line touches.  When this breaks,
the shaft flops around and the eccentric sleeve on the end of the shaft that
presses the reverse light switch cannot operate the switch properly.
Shifting problems result and the reverse light comes on at inappropriate
times.  In the case with my transmission, the reverse lights came on when
shifting between first and second gears.

You can take the "locking cover" (see "cover" on diagram on Bentley 35.60a)
off and feel the "stop cap" move around if it is broken.  When the cover is
off you will also be able to tell if the switch has come apart like Scott
mentioned.

Although the"Stop Cap" is listed as a separate part, it cannot be replaced
because the eccentric sleeve cannot be easily removed and replaced without
damage.  I purchased an entire used selector shaft from Daryl at AA
Transaxle and had it fixed in under an hour.  If you take the "gearshift
lever" (251 711 221 F) and the "locking cover" (091 301 232 A) off the
selector shaft will slide out and you can slide the replacement in with no
problem.  These part numbers are for a '86 transmission.  Your numbers may vary.

Ed