Here are a few simple tricks for car-camping in the rain. Most of these may be well-known to people who camp in the wet... but for those new to it, here are some little lifehacks we've put to use in various state parks here in the northwest.
When car-camping in the Pacific northwest, the first order of business is often to get the tarp in place.
To start, tie a rope around a tree near your eating area. A good knot to use is a bowline knot ("the rabbit comes out of the hole..."). You can also use a "no-knot," in which you simply wrap the rope a few times around the tree; friction will keep the rope in place.
Keep the tarp high enough to allow good light in. Use a forked stick, or a "chuck-it" ball-thrower if you have one, to coax that rope higher up the tree.