Fiest 2011 Blower issue, looking for ideas

I bought a 2011 Fiesta with 167,000 km about a week ago. When I tested it before buying, I noticed the blower motor wasn’t working. I didn’t think much of it at the time and assumed it was just a bad motor or something simple.

I ran a series of tests, but I still can’t pin down what’s actually going on.

I started with basic multimeter checks. With the blower motor disconnected, I measured ~12 V at the blower connector, so I initially assumed the blower motor itself was faulty. Before replacing any parts, I also checked the relevant fuses multiple times and swapped the blower relay with another identical one several times, but this made no difference.

After a few days of driving while waiting for the replacement part (using a USB desk fan to prevent windshield fogging), the blower randomly started working 2–3 times. When it did, it worked normally for a few minutes and then shut off again. These events didn’t correlate with anything I was doing while driving. The only constant was that the HVAC panel was left ON with ~60% heat. This intermittent behavior made me suspect a wiring or control issue, so I wanted to confirm that before replacing parts.

To validate that the car-side wiring and controls were capable of powering a blower, I disconnected the connector from the original (supposedly faulty) blower, attached two temporary wires to that connector, and used them to power a new blower motor directly. The goal was to verify the wiring and control path before committing to the ~4-hour job of replacing the blower. In this configuration, the blower ran correctly, so I concluded the wiring was OK and the blower motor was the problem. I also recorded a video of this test as a reference.

Based on those results, I replaced the blower motor. I also replaced the blower resistor, mainly because it looked degraded, even though the wiring diagrams show that speed 4 bypasses the resistor entirely.

Despite this, the blower still didn’t work.

After the replacements, I went back and did more careful measurements. I then noticed that with the blower disconnected I still measure ~12 V at the connector, but as soon as the blower is connected and draws current, the voltage drops to 0 V.

This strongly suggests that the circuit shows voltage with no load but collapses under load—i.e. voltage is present, but there is no usable current.

I considered a grounding issue. Tracing the ground path back toward the HVAC control panel is difficult without removing the dashboard. I did try temporarily adding ground wires (alligator clips) from the fuse box ground to the battery, based on reports of oxidation issues at that connection, but this had no effect.

At this point, I need advice on what to check next—ideally without removing the entire dashboard.
I’m mainly looking for ideas or direction on where to go from here.