The right tool makes the job easy.
1. Wire tool. You know those yellow handled jobs that you get from CTC for $9.99 with 100 connectors? Toss em'. Cars have wires, lots of em. If you plan on doing any serious car audio work (even as a weekend enthusiast), get you some good ones. I have a set for crimping and a seperate set for stripping...

These are the strippers I use...best ones I had were from Rat Shack...nice and cheap...

2. Drill. I like my cordless drill...hell, it borders on love. A good cordless drill will be one of the handiest tools in your box. I use a 12V cause they are a bit lighter than the 18V out there and they still have good torque. Get a keyless chuck and make sure you have extra batteries. Nothing worse than a cordless drill with a dead batt. Corded drills aren't as convenient but are good if you get into building lots of speaker enclosures which can wear out the batteries on a cordless pretty quick. Spend money on Makita if you like but I've had my trusty Mastercraft for a couple of years now and it still works great.
3. Screwdriver set. Good for removing everything from door panels to TPS sensors

. Invest in a good set. Cheap ones break or strip. Make sure they are comfortable or you'll blister your hands after prolonged use.
4. Socket set. Again, avoid the cheap sets. You can go to CTC and get a set just as good as the snap on stuff. It also has the same lifetime waranty. You will want Metric and Standard. 1/4", 3/8",and 1/2" drive. Remember to put the sockets back after you use them

5. To go with your socket set you will want a nice wrench set too. They can reach places that your socket set can't. They new stubby wrenches are very handy too. Again, don't skimp here. Nothing like forcing on a bolt and having a tool let go and destroying your knuckles.
6. Digital Multimeter. Good for diagnosing a host of electrical problems or doing measurment for stereo type stuff. Again, you don't need a Fluke. If you are gonna go hardcore then get it...they are definetly the best out there...you will pay a premium though.
7. Pick set. Good for popping out panels and probing behind them to know where the clips are. Nice for popping out speaker grills.
8. Knife. handy to have. Not gonna go into detail here...I'm sure you an think of a dozen things on your own.
9. Soldering. I have a gun and an iron. I use the gun for soldering speakers and 1/0AWG cable. Does it all. I use my iron for delicate work like relocating displays and such.
10. Torx, Allen and Star set. These can be used to undo seat belt bolts to panels to distro blocks.
11. Dremel. Very good tool to have for detail work or porting your intake for an enlarged TB. Don't go cordless. Variable speed is best and getting a flex shaft is a good add on as it's very useful.
12. Mini flashlight. Can't work if you can't see. Some places under your dash are DARK...live with it...get a mini flashlight. Mag Light is what I use and will never look back.
Now if you want to do more specialised things with stereo stuff or other fabrication...
1. Jig saw. Get a good one with veriable speed. Pay decent money for one 'cause it sucks when they burn out oin the middle of a project.
2. Router. Best tool for finishing wood and cutting out perfect circles in MDF. Get something with decent HP so that it doesn't bog down on thicker material. Get one with a vacuum attachment. They can make lot's of dust.
3. Clamps. Best thing for holding glued projects together or metal before you weld. Also makes a nice chip clip.
4. Palm sander. Nice for finishing enclosures. Saves heaps of time over hand sanding. Go a step up and get an orbital...much better.
5. Butane iron. Soldering guns can put out DC current when the tips break. This can ruin sensitive electronics. Butane irons solve this prob;em.
6. Propane torch. I use my gun to solder 1/0AWG to terminals but as soon as I have a house I'll be using a propan torch. MUCH faster and better overall.
7. Vice. Holds things sturdy while you work on them. Have you ever held a piece of cable while you soldered and end on it and burnt yourself? Get a vice. Als good for crimping large diameter cable.
8. Grinder. Good for cleaning metal (with a wire brush) or shapind plastic. Nice for sharpening stuff too.
9. Air stapler. Great for working with fiberglass. Best for stapling fleece to frame work before you lay the resin.
10. Heat gun. Nice for speeding up the curing of fiberglass or softening vinyl for door pods.
11. Table saw. Best/easiest way to make straight cuts in wood. get one with a vacuum attachment. MDF dust tastes like shit and isn't good for your lungs either.
12. Proper work bench. My last one was made with a 1" MDF top. Didn't feel guilty about messing up the top 'cause it's easily replaced. Make sure it's sturdy and can hold lot's of weight.
There are many more that I'm sure I'm leaving out here...this is more of a basic guide and will get the ball rolling with other members input...