Ihave a 2000 gmc sierra that has a oil leak down

Likely the reverse switch. This is either on the shifter assembly but more likely it is screwed into the transmission. An easy swap after correctly identifying the switch. You may lose some transmission oil so put a pan down.

if it is slippery and stinky, your heater core is leaking. otherwise you have a leak around your windshield and rainwater is coming in and not drying out as it is under the mat. very hard to track down, get some clear aquarium sealer and get to work.

best way to figure it out is by checking your current fluid levals. thats a good way or narrowing it down

Between the rear of the block and the transmission is just a couple of things that may be leaking, but make sure thats where it's coming from, and not running from somewhere else...
Your crankshaft rear min seal, they get hardened after yearsm and some can cut a small groove in the end of the crank and leak. They make a "speedi-sleeve" to repair that. The other idea is the camshaft plug, which is like a freeze out plug, but these are not as common.

sounds like a switch as common error oboth sides unlikely

Under the truck....under the drivers feet area.

piggy back to the starter, right hand side of the motor looking out of the windshield from the drivers seat... down low underneath next to oil pan... two mounting bolts, it is a gear reduction type starter...

THis would be way too much to walk you thru. If you like to do your own maintenance-buy a chilton manual online or at your local auto parts. It will save you time and $

check owners manual,or call a gm service dep.

i've seen alot of those (4L60E) leaking from front pump