This answer is based off knowledge of my '95 skylark, take it with a grain of salt because it could be different. Should still be accurate because GM stereos differ little from the late 80's to mid 90's. (My brothers old '89 Pontiac grand am had the same setup)
I believe it has a double-din car stereo, low wattage. May or may not have a tape deck (depending on factory options and such). Should have 6x9 delco brand paper cone woofers in the back (complete with whizzer cone! lol) and similar 3x5 speakers in the front.
a 1991 buick skylark has 4" speakers in the dash and 6x9's in back
how do you replace a ignitionswitch tumbler 1991 buick skylark
The thermostat on a 1991 Buick Skylark is located near the engine. To find the thermostat, follow the top radiator hose directly to the thermostat.
where does a clutch relay seleniod go in at on a 1991 buick skylark
I'd say about 2.5 tons.
If it has a coolant tank, that's the "cap".
You can find the 1991 Buick Skylark radiator drain plug on the bottom of the radiator. The radiator drain plug will be in the center of the radiator bottom.
Under dash with fuses. Very hard to see.
The PVC valve for a 1991 Skylark should be located on the top of the valve cover. It is a plug in type, twist and pull to remove.
Either a 2.3/2.5 4cylinder or a 3.0 V6
a round one that you can order online
Probably a worn-out or clogged heater core.