Why would the heater on a 1997 Chevy Suburban work at all speeds except high and the back blower not come on at all?

The two blowers share two things in common that are worth checking, the fuse (#12 on the interior panel) and therefore powersource, and the Hi-blower relay (3rd from the bottom-right of the under-hood relay/fuse block). Before continuing, however, it would be helpful to know the exact combinations of working/not-working for your problem. For instance, Front blower working off: Back blower Low - works Med - works High - works Front blower on low: Back blower Low - works Med - works Hight - fails Also, with/without the A/C on MAX, and the A/C on/off at all. You get the idea. Now back to my original comments... If you can test the scenario(s) at night when you have darkness and can examine subtle lighting changes, like the dome lights that would be good. Assuming that you don't have significant lighting changes when moving the switch(es) to different settings, you probably don't have a "short" problem. Note: a short may not always lead to a fuse blowing, but may have led to a burned switch and/or wire, motor, etc. It really sounds like you have two different problems. The "Hi" setting on the working blower and a fail-to-run problem on the other. The first thing to do would be to double check the relay I mentioned above in the under-hood panel. Once that's done, try the following steps to isolate the issue. Do: Remove the power lead to the rear blower and try the front blower in all positions. Then: If it works fine you know it can on it's own. Do: Plug the rear blower back in, and retry the front blower. Then: If it doesn't work, you probably have a rear motor burning the relay. Check it again. Also: If it works, you've shown the rear motor doesn't have an effect in the off position. Try turning the rear motor to different speeds and see if the front motor fails (while at high) in any power-on settings for the rear motor. Also: If not, you've shown the two to likely (99%) unrelated. For the rear motor... Do: Disconnect the motor and use leads to run the motor straight from the battery. NOTE: You should connect the leads to the battery and the motor with different leads, so that your final connection is with alligator clips or such. The final connection that powers the motor has the potential to burn the connection point. You don't want it to be the terminals or vehicle connections. Also, make a solid connection with waivering, then a solid release if it turns or does not turn. It will only take a second to check it. FUSE THE LEAD YOU USE!!!! use a lead that has an inline fuse with amperage to match the factory fuse in the fuse-block!! Then: If the motor turns, you will have to chase a connectivity issue (known as an open) throughout the system. Also: This will be best accomplished with the power off and an ohmmeter between the fuse block and all the locations you can imagine. Start with the front-upper-console, then the rear-upper console, then perhaps the lead at the motor. While checking the connections, move the switches to various postions to ensure you have some/none success in an "willing" position. The one place you should have success with the continuity check is the front-upper-console, in one of the various power positions. GOOD LUCK!! I'll check back. User - Atis