A couple of random comments...
I've camped and driven along the California coast in winter a couple of times. It's very doable, but the weather may be blustery and cool. Most likely you'll see damp foggy mornings with a bit of mist in the air, clearning to scattered clouds or perhaps bright blue sunshine in the afternoon. You *can* see temperatures in the 80's in December, but most likely it'll be in the 60's -- and there is a reasonable chance of rain in December. All this of course, is dependent upon the year and the specific winter storm patterns going on -- so it just means that you need to be prepared a bit.
Depending upon where you go in Mexico, you'll get different advice I think. If you just want a flavor of Mexico, take the light rail system in San Diego down to the border, and walk across. You can catch a taxi to the Plaza de Revolution and explore the touristy places there a bit, before heading back. That gets you out of the hassles of Tijuiana (TJ) traffic, and the foot crossing is usually quite a bit faster than by car.
If you want to see more than the "touristy" TJ border town, you should go down to Ensenada (about 2-3 hours drive south of TJ). It's been built up into a resort community, but doesn't have the same border town feel. The road is very good (its a toll road), highly patrolled, and not too far. But to get to the toll road you have to drive through TJ which can be an experience in itself. Make sure you check to see if your rental car company allows you to take the car across the border, and that you have appropriate Mexican auto insurance (and health insurance, just in case). If you don't, you can end up getting held in a local jail for a fender bender. If you're set on going across via car, and your US rental agency doesn't allow cars into Mexico, you can cross over and rent a car in Mexico -- there are several national level chains which operate there (Avis and Budget, if memory serves, among others). But again, make sure you have the appropriate insurance and paperwork with you.
As an alternative, give some thought to a flight from San Diego to Cabo San Lucas at the end of Baja or even Mazatlan or Puerto Viarta. I visited them last Christmas/ New Years and found the weather very nice (warm, sunny), and enjoyed the trip. These would be either a flight, or there's several cruise lines which operate a week long cruise visit down to these three cities from San Diego or LA area. Cabo is more "beachy" (beaches, resorts, sport fishing, kayaking), Mazatlan is more fishing and restorts, and Puerto Viarata is more mainland old Mexico for a place to visit. All enjoyable.