Here's a breakdown of the likely culprits:
* Internal Regulator Switch: Many rear window regulators have a built-in switch that's activated by the movement of the window. This switch tells the motor to stop when the window is fully up or down. If this switch is faulty, the motor might only receive power in one direction. This is the most common cause.
* Wiring Issue (Less Likely): While you've tested with another switch, there could still be a break or short in the wiring specifically related to the "up" command. This is less likely given your testing, but a possibility if the wire is damaged near the regulator itself and not further up the circuit.
* Damaged Regulator Gear or Cable: Though less probable given the symptom (power only in one direction), it's possible the mechanical components of the regulator (gears or cable) are binding or broken, preventing the motor from being able to easily lift the window. If the motor is struggling against a mechanical problem, it might trip a safety mechanism or draw too much current, causing the upward power to be interrupted.
Troubleshooting Steps:
1. Inspect the Regulator: If you're comfortable doing so, remove the interior door panel (plenty of online guides exist for this). Carefully examine the regulator mechanism. Look for any obvious damage to the gears, cable, or the small internal switch mentioned above. Listen carefully for any unusual grinding or binding sounds when you try to manually move the window up or down.
2. Check Wiring (Advanced): If you're comfortable with electrical work, you could check the wiring leading to the regulator for any breaks or shorts. This is more complex and requires some electrical knowledge and tools (multimeter).
3. Professional Repair: If you're not comfortable with the above steps, it's best to take it to a mechanic or auto electrician. Replacing the entire regulator is often a more practical solution than trying to repair a faulty internal switch or complex wiring issues. The cost of a replacement regulator is usually less than the labor of a more involved repair.
In summary: The problem is almost certainly mechanical within the regulator itself, specifically the internal switch or a related mechanical failure hindering the upward motion. Directly inspecting the regulator is your best starting point.
