What solder to use on basic automotive wiring?

Take a good look at post #2 and #12.. Those are the type of pliers you need to use , and how to use them, if you want guaranteed results. The el-cheapo crimping pliers that often fail are pictured below.

As far as soldering your wire connections, I would advise against it. Use some good terminals and a decent set of crimpers and you'll have great results! Soldered connections, as far as I know, are not used in any vehicle from the factory. That goes for RV's as well.