What solder to use on basic automotive wiring?

Maybe invest in better crimps? I've used these forever it seems. Never had an issue with pull out of solder less connectors.

http://www.grainger.com/product/STA...m/rp/s/is/image/Grainger/3KH45_AS01?$smthumb$
I have a pair exactly like that; it is a great crimper. You can find crimpers like that at places like Lowes but the quality in the jaw alignment is variable; you have to be picky on which one you actually buy. Also make sure to align the crimper jaws so that the 'nub' that pushes into the crimp sleeve is always on the opposite side from the split in the crimp sleeve.