1992 Toyota Celica GT engine vibration causes?

I have personal experience on this one! My car shook me like a ragdoll when sitting inside my car at stoplights! It also caused severe vibrations through the seats and headrests while stopped and while going much of the time. I was told by a local mechanic it was the right motor mount. Another mechanic looked at the right motor mount, while we had it out for another job. He thought it looked all right, so it went back in. After a couple of the other motor mounts were replaced, the engine noise was much less, and the seats and car frame vibrated a lot less for about a day---then it shook like before. I ordered a right motor mount and replaced the mount myself, and that made a HUGE difference in the noise the engine made, and in the seat vibrations. My car still vibrates at dead stops, which leads me to believe that the transmission-side mount (opposite the right motor mount and the other of the two most important engine mounts) never was replaced--though a mechanic claims he did the job. It hasn't been all that bad, so I haven't bothered putting the money into it yet, but a 1992 Celica with bad engine mounts can make your car twice as loud as a car with a bad muffler, if not three times as loud---where you actually can't really listen to music in your car comfortably and your head practically bounces off your driver's seat headrest and you feel vibrations that feel more like an earthquake going on!!! If you have your engine basically running on three spark plugs and with no catalytic converter on the car on top of that (like I did at one time).--yes a 9.5 on the Richter scale!! Bad motor mounts can cause great catastrophes in your car, including loosening your transmission to the point of falling out, if you wait long enough (I didn't experience this, ok?! But it can happen!)