Auto electrics, wiring and charging faults
A vehicle that is flat every few days is usually not the battery. It is either something draining it overnight or a charging system that has stopped putting the charge back, and a new battery just buys you a fortnight before the same morning happens again.
Charging systems tested before anything is replaced, so you are not buying an alternator to cure a corroded earth strap. Batteries, alternators, starter motors, glow plugs, lighting, sensors, instrument clusters and the looms that join them up.
Auto electrics is the half of this trade most mobile mechanics leave alone, and it is on our own list on Facebook for that reason. If somebody has already had the dash apart and given up, that is still worth a phone call.
