The front 2 speakers on my car have been replaced few times already.
Because of this, the upholstery (cardboard backing) is not very good,
and has little strength to hold another speaker.
Any idea how I can remedy this? Are there good, durable speakers
available for 1993 240 ?

I ordered 4" Kenwood speakers from Crutchfield.
http://www.crutchfield.com/p_113KFC1062/Kenwood-KFC-1062S.html?tp=102

They fit really good, give better than average performance, and when I got mine, Crutchfield included a plastic ring (for free) that you install behind the cardboard for the screws to attach to.

As with all Crutchfield orders - installation was a snap as they detail the steps ad-nauseum.

(BTW - those same speakers fit my rear as well - made a really nice set).

Oh - one more thing. Someone else on the Brickboard used the same speakers and they had manual windows. They said that they needed to mount the grill updside down to keep the window handle from rubbing. Since I have Power windows, I had no such problem.


Hope that helps
Charles

Difficult problem to solve. I think I finally cut a reinforcing ring out of hardboard, or possibly glued some washers to the other side of the door panel. I can't remember, anymore. Be creative, and good luck. It's a common problem.