Hi, you must have got it fixed or maybe no longer have the car but in answer to your question; the solder connections are inside the mas relay where the inside of the pins are soldered to the board or relay components.

The solder points can develop small cracks that open the circuit (shutting off power) They cause intermitent problems because at certain temperatures they are connected, but at other temperatures and humidity the crack opens creating a gap, stopping current (voltage) flowing and causing stalling.

take care,
Acky