As my esteemed colleague flyng_fool stated " get a cheap multi-meter and check for continuity across the fuse".
But I would guess if from what you just mention that the bulb is good because all the tweeters run through it. In other words, if it where blown, you wouldn't hear any of them working in any position.
The two rear tweeters run in series with a single resistor. They pick up just after the bulb but before the time delay circuit. The 1.8 ohm resister is just before the first rear tweeter which is then connect to the second rear tweeter. Don't confuse it to the second 1.8 ohm resister which is also right after the bulb but goes to the time delay circuit for the front tweeter.
If you look at your photo of the crossover, the brown lead should be the positive for the rear tweeter with the blue being the negative lead. The 1.8 ohm resistor for the rear tweeter is the one under the bulb closest to the spade connectors.
Because they all run in series, if either of the tweeters or the resistor is bad, or if you have a bad connection, nothing will work.
So, if the front tweeter position is the only one that works, it means that there is a problem in the rear circuit. Use your newly purchased multi-meter and set it to check for continuity.
The easiest way to check the wiring and resistor (I would assume they're both good) is to remove both rear tweeters and tie the leads together (remember which leads go to the positive and negative) . Remove the blue ground wire from the circuit board. Check for continuity by touching the leads to the end of the blue wire and to the end of the light bulb closest to the spade connectors. If it's open, you have a bad connect or wire somewhere or bad resistor. If that's good you should just need to replace the bad tweeters. Ebay is your best place to find them. DCM used that tweeter in a lot of their later products. I you can't find any used DCM tweeters, use the good ones that you have in the front because the dampening circuit is designed for the modified DCM tweeters. The rear ones would not be as critical.
Here's a copy of the schematic, for reference. R3 is the resistor for the rear tweeters.
P.S. Also, change out the electrolytic cap on each board.Edited by Jamie Hauser - 1/28/13 at 12:04pm