Ok, so I tried hooking up one of those radiator back-flush kits, and I think I put it on the wrong spot. After reading the directions, it sounds like it hooks up to the heater hose that goes INTO the heater. After looking at the workshop manual, it looks like THAT hose is behind the rear rotor housing. Well, I just took a photo, and the hose on top says "fuel line." Is it the hose on the bottom?

 

Okay, n/m, the heater hose is actually there..in the background.



I also found a great write-up here: http://www.rx7club.com/archive/index.php/t-485756.html



...in case anyone was interested. I plan to try this out in the morning.