Anyone know where the thermostat is? I am going to change it and I don't have a clue where to start. I heard it's an easy DIY job though? Please advise, thanks guys.

Follow the yellow brick road
 

Hott what? Ha Ha.
Follow the upper radiator hose from the radiator to the engine.
Where the hose is clamped to the engine, at that round sort of a thing. Probably held in place by three cap screws.
You've just laid your eyes on the thermostat housing.
That wasn't so bad was it?
Happy Trails Beep Beep from Houston!!!
Donald, El Cheapo

Thanks Donald, you're the best. Also, Ali told me something about have to bleed the system before I finish? I don't feel safe about bleeding anything! Do I have to do this? Do I need any tools for this?

Burp that Baby
 

Yes you should bleed the air from the high places in the cooling system.
You probably could do it by removing a high heater hose.
But since you are replacing the thermostat, leave it out and fill the block at the thermostat hole. This will completely fill the block.
If your's is an inline config, there is a plug at the top of the cylinder head, remove it to bleed the air.
But as a last resort, just fill it to the top, wait until it burps and fill some more. Keep this up until it doesn't burp anymore then start the engine and add coolant as required.
You'll get him filled with no danger to you or him.
Happy Trails Burp Burp from Houston!!!
Donald, El Cheapo

Don't forget to drain the coolant out of the radiator and the block before you open the thermostat housing to avoid a giant mess.

Hi, David:

No, you don't need to remove a heater hose to bleed the coolant system. Here is Benzmac's opinion of how to get the air out:

Benzmac wrote on 05-20-1999:
======================================================
Yes the drain plug on the block is located under the exhaust pipes on the under side of the block. Be sure the installing new coolant be sure to try to overfill the reservoir this is the only way to purge the coolant system. After the system is completely full, turn on the defrost and crank the engine. If you don't get heat out the top soon, turn the engine off and add more coolant.(Until it pours out the top of the reservoir)
======================================================

Or you can search "Replacing coolant for 94' C280" for the entire thread.

PS: Thanks for letting me know the shop in Huntington Beach. I did get a check engine light last week. (See "94 C280 cold start problems..." for detail) What a coincident. I haven't have it checked yet but at least I have the peace in mind of knowing where I can go to. Thanks.

Hottee 18m
 

Read all of the posts.
There are some operations that are so obvious that they go without saying.
Use your knoggen(grey matter) and do your thing.
This is in hope that you haven't blown your cooling system to bits.
There is an old Engineering formula, K.I.S.S. that applies to all instructable things.

"K"eep "I"t "S"imple "S"tupid
Happy Trails Beep Beep from dumb de dumb dumb Houston!!!
Donald, El Cheapo