2000 focus electrical surge

my girlfriends 2000 focus’ lights, fan, etc are surging when motor is under load. voltage regulator? alternator?

Maybe it’s not a surge but a dropout? Start with the cheap, easy and free stuff first. Check both battery terminals and the chassis ground. Next, stop at an Autozone or similar and have them do a battery load test and charging system test. Likely you’ll find the problem somewhere along that diagnostic path.