How did the Libians find Doc?

This is not 'knowable' information, as it's never explained in the movie.

However, if I had to guess or try to figure it out, I'd say MAYBE the following chain of events could be at least feasible.

1) They knew who he was, perhaps someone followed him home just to make sure - after all, they wouldn't make a huge plutonium and bomb-deal with some stranger they haven't thoroughly checked out and know all about (though then they SHOULD know about the time travel experiment, too)

2) They realized they had been betrayed by him, sent troops to 'find him and kill him', as the saying goes. These troops arrive to his known address, but Doc is not there. They rummage everything, they find pieces of paper that point to his plans, and eventually figure out the Twin Pines Mall stuff. Perhaps Doc recorded all his phone calls, so they can hear his voice talking to Marty about Twin Pines Mall and the time as well.

Alternatively, they have a map of the city, knowing about the time travel experiment, they simply use logic to deduce the parking lot of Twin Pines Mall is the only place feasible for doing such an experiment (possibly the only big enough, good quality flat area like that in town?).

For the purposes of the movie, it doesn't really matter -how- they found him. It might be that Doc wasn't trying to keep it a secret, it could be that they were simply driving around after realizing Doc is not home, and then saw this massive truck in the parking lot and decided to check it out, and simply hit the jackpot that way.

Seriously, if you know the town, it's not that hard to find them - Hill Valley looks to be pretty small and there are not many places he could be within it.

Better question is, why were they so weird with their van lights? If I remember correctly, they first had the lights on, then turned them off, then turned them on again. What kind of sense does THIS make?

he was probably still in the phone book :.

GPS tracking device.