Might be that the alternator just isn't man enough to run the headlamps and heated seats all at the same time.
Similar happened to me last weekend on MTWC Night Trial. Ran full beam most of the route and intercom and 12 volt electric blanket. When we stopped at the last control point and switched off the engine it took a push to get us started again. Ran to finish on dipped beam (or side lights where we could) and got some charge back into the battery. Volt meter read 12.3 on dipped beam. Morning after with no lights on it was reading around 14.5 volts and the battery charged ok.
Isn't the M3W be running twice the number of headlights, never mind accessories, than a motorcycle, which maybe is what the alternator is designed to normally power?
