The trans switch is the ground for the light, The 12v source comes from a connection under the dash,
SO,
test #1 if you have 12v at the light connector with the key on, (think test light) step one is complete.
If not you now know why the light isn't working.
if you do,
that would indicate You either have a connection issue between the trans and light (IE: ground) or the trans switch is not working. If you jumper the output wire at the trans pigtail to ground and the light works the trans switch is bad or the linkage is misadjusted not allowing the internal lever to fully activate the switch,