There's a seller on Ebay now selling adapter plates for 6.5" speakers to go in the doors of an S13. They're made from 1/4" thick PVC sheeting. I bought a set, and they pretty much fit perfectly; they exactly mimic the outline of the stock yellow plastic mounts, and have three sets of holes for whatever speakers you get.
They cost about $12, and I highly recommend them, they made my install go very easily.
Here's a pic on the passenger side, with the Alpine speaker I used:
