Basically you are redesigning/modifying the electrical system. This means you need to do a bit of "automotive engineering" if you want to avoid fuse blowing and electrical problems in the future!
I'll tell you the right way to do this.
When they design these circuits at the factory, they take into account the amperage needed for various devices, then use the correct size wire to handle the electrical load for that circuit, and use a correct size fuse to protect that wire.
The fuse makes sure the wire will not get warm or hot and possibly cause a fire or melt. So the fuse is there to protect the wiring and the vehicle.
Generally they design circuits to handle the load and not much more. In some cases there is a bit of extra amperage capacity in a circuit you could use without blowing the fuse.
The way to "figure this all out" is to use a factory wiring manual which would be a part of a factory service manual set. And an amp meter.
You would determine what all is tied into a particular circuit by looking at the factory wiring diagrams, then turn on all those devices. Then measure the amperage being used when all those things are turned on.
So say there is a 15 amp fuse and this goes to the fan and the radio. You turn the fan on high and also the radio, then measure the amperage being used on that circuit. (The most it would ever use.)
Say it is using 10 amps. That would leave 5 spare amps. However it is best to only use 80% of a circuits capacity. So 80 percent of 15 amps would be 12 amps. So this circuit would have 2 spare amps!
Then next connect to a battery the things you want to add to your vehicle and measure the amperage they use with an amp meter.
If the devices you are adding use below 2 amps total, then you could safely add those to your circuit and not worry about having a fuse blowing problem.
Note: The 10 amp capacity amp meters they sell in auto parts stores are useless! If you measure more than 10 amps with these, you will wreck them! Get a 200 amp capacity amp meter and you will be able to measure everything on a modern vehicle including a 100 amp alternator which some vehicles have.
Here is a good amp meter...
http://www.testequipmentdepot.com/ex...ters/38394.htm
Notice here on the data sheet it says "DC Current 600A"!
http://www.testequipmentdepot.com/extech/pdf/38394.pdf
Some meters will be 200 amps AC, but only 10 amps DC. A car is DC or direct current, so read the fine print before buying the meter!
Here are wire sizes and the amperage these will safely carry...
Wire gauge sizes for 12V wiring (10 Ft. length)
18 - 5 Amps
16 - 10 Amps
14 - 15 Amps
12 - 20 Amps
10 - 30 Amps
6 - 100 Amps
4 - 150 Amps 
If you want any pictures of the connection locations let me know i'll take a pic ASAP to double check it's right. 
