1989 Blazer end gate issues
 

I don't know how that window works specifically, but that safety switch is most likely normally closed, and opens up to prevent the window from operating. Maybe buzz it with a meter, see what position it's in? If open, I would try to jumper around it, see what happens.

Also, have you checked voltage at the motor while someone operated the dash switch?

Again, I don't know anything about first gen​​​​ K5 (?) rear windows, but I've got a pretty good understanding of basic electrical theory

The switch is only there to remove the possibility of the glass moving up into the closed position with the gate down. It functions like cleburne red described it by completing the circuit when the gate is closed and opening it when the gate is down. It is VERY simplistic and easy to check with a multimeter set to test continuity. Should have near zero resistance when the switch is closed and should read open when the switch is open.

As far as bypassing it, yes it can be bypassed either by using a jumper across the connector or by cutting the connector off and using a butt connector to complete the circuit. Many people have done exactly that over the years with these trucks in attempt to get the power rear window working again. Most skip over the actual electrical troubleshooting steps and jump right into replacing parts like you as well.

My '74 has a manual regulator into it now, but I have a power regulator that I plan on putting into one of my rust free gates when I get some time. When I do that, I am completely redesigning the circuit to include relays back at the gate to control +12V to the motor. I will also be installing a good ground to the inside of the gate as well since this is a major concern with the older gates. Without pulling up a wiring diagram for the newer ones, I can't say for sure if they ever remedied the issue of ground being transmitted through the hinges as it was in the older ones.

Let me see if I can find a wiring diagram specific to your truck.

*EDIT* - So here's what I could find that should match up with your truck. It came out of the 88 R/V (K5 Blazer & Suburban) wiring manual that I have, but it should show you where you need to be testing and for what. It does look like they did add a dedicated ground to the gate, but you should test for good continuity to the body and/or directly to the battery negative post. From there, you need to see if you have +12V when one of the switches is activated. The light blue wire should have +12V when the switch is moved to the UP position and the tan with white stripe should have +12V when the switch is in the down position. As the schematic shows, the light blue wire goes through the cut-out switch to prevent the window from going up when the gate is open.
https://cimg0.ibsrv.net/gimg/blazerf...2c4d137c7c.png

Thank you sounds lik it's time for the multimeter.

It shouldn't be a drain on your battery. The dash switch would still need to be operated in order for the swtich to be completed, the safety switch being jumpered would just allow the circuit to be completed in its entirety.