Car Stereo Problem

OK, so I know very little about car stereos, and am having trouble finding answers on the web, so I will throw this out there.

My front speakers are not working. What happens is that sometimes, without warning, they will just die out, so only the back ones work. Then they will return sometimes without warning. Sometimes the front speakers make a hissing/static sound when they are either coming or going. This was a problem years ago, but went away, only to recently return. (NO it is not the fader on the stereo, like I keep telling my friends who think that is the problem)
I asked once at a stereo place if it might just be a connection problem, but they said I needed a new amplifier. Anyway, the problem went away for several years, so I didn't worry about it.
I wasn't really aware that I even had an amp, I thought I just had the reciever/tapeplayer and speakers. Do all cars have a separate amp? And where is it located in your typical SUV?

I have a 1994 Nissan Pathfinder, I got it in 1997, with factory 8-speaker stereo system. Also an FM/CD changer, but that was installed after the problem, and doesn't seem to either cause or not cause it. It is pretty nice when it works, but the no front speakers is driving me mad, and I head out for a road trip this week.

Today, i took out the reciever and looked at the connections, but they seems fine. I can't tell where the wires go to once they get under the dashboard though, so I don't know about if there is an amplifier under there. IT is quite confusing, and I have no documentation about the stereo (bought it used.)

Mostly, I guess I am looking for advice as to whether I need to have a stereo place replace the reciever or amp, or whether I might be able to troubleshoot the problem myself. Money is pretty tight, so I don't really want to have to pay for anything unless I am certain that is the problem. I don't trust car-repair folks anyway, one to many times they have told me what needed fixing and a price, only to have me do a little work and fix it myself for a fraction of their quote.
Anyway, any help on what I can do to try to isolate the problem would be helpful. Thanks in advance.

Thanks guys

Wow, thanks guys, I feel much better about all of this and the possiblity that I might be able to find the solution out before replacing everything.
Bsmith, it sound the same, yes the woofers are down in the door, whith the tweeters in the pillars, and then the 4 back speakers are overhead. Sounds similar to the 95.
OK, so I will start with looking for these amps above3 the wheel well. IT seems that I heard this before somewhere (that the amps were in the back near the wheel, maybe that is what they told me several years ago.) Anyway, good idea to test it with another deck, so simple, yet I hadn't thought of it.
Nice to see that i am not just being paranoid about sales folks. Went in to a shop today to inquire, and my friend had a problem too. I didn't learn anything, but when my friend asked the sales guy how much it would cost for them to just look at his expensive deck that had died, the guy was like '$100 just to look, then possibly several hundred more to fix it. You would probably be better off just getting a new deck'. So we went around to the service place and asked the same question: '$35, but they will get you an estimate before billing anything.' That is what I hate about sales, it brings out the worst in people sometimes. Anyway, I gotta go on vacation, so I doubt this is going to get fixed before then, but I will let ya know after I get back next week.