1994 Seville STS Northstar--where is the thermostat? Upper Radiator coupling on motor? Does it pop right out? Also--is it necessary to have a thermostat for warm weather driving? thanks!

It's where the lower radiator hose connects to the water pump housing. A thermostat is needed year round. You can get a Stant #14228 at most auto parts (advance/auto zone) or an AC Delco #131-128 (GM#12559807). They are identical parts, Stant makes both. I know because I bought both to compare. If you buy the Stant it comes with the rubber gasket, the Delco doesn't. To replace you need to remove the hose and remove the two bolts on the thermostat housing with a 10mm socket and ratchet. A cool engine is preferable.

Also--is it necessary to have a thermostat for warm weather driving? thanks!
YES.