why are only a few components working?

Ill start off by saying, i know very little about electronics. I am building a dc circuit for my car. I have 6 switches, one that is for the main power and has a big green led next to it so i remember to turn it off. The rest are so i can hardwire my GPS, Radar detector, and fog lights. I also have two switches i can utilize later on if i want to add something. I tested it on my bench using a portable car jumper. checking each individual i got all 12.91v. with all the switches, but nothing connected, i got 12.89. Then i hooked up the radar, nav, and fog lights. All of them worked. While they were on, i measured one of the open switches and was getting 11.89. Yesterday i brought it down to my car to install it. i measured the battery leads, and got 12.41v. Then after hooking it up, the Nav and lights don't work. The radar come on and works, but when i flip the switch for the navigation, the radar will go off and come back on. i have a 1.5a inline fuse for the radar, and a 15 for the lights, as recomended. The Gps i simply bought an auxiliary 12v outlet and used the normal power cord that cane with the GPs. All of the fuses are secured behind the knee bolster and come up through the dash. I also have a 30 amp fuse on the main positive lead. i will post a diagram when i get a chance, but the positive lead comes from the back of the car, goes into my main power switch. from there it goes to a dist. box and to each of the switches. out of the switches they go to their fuses then the component. the negative does the same this, just without the switch. But it only goes to radar and nav because the lights are grounded to the chassis. Any advice or suggestions are welcome, ive never really messed with electronics like this before, and its turned into a month long project that i want to finish

***I drive an 06 mini cooper s** so the battery is in the back, the main leads are 10 guage, and the rest is 18 guage

It sounds to me like it's time to find a professional to finish this project."Always listen to experts. They'll tell you what can't be done, and why. Then do it. - Robert A. Heinlein"

That requires money, of which i have none

I can understand that. If you can be more specific I can do my best to help... Will get back on this when I get back home. PM me if I don't!"Always listen to experts. They'll tell you what can't be done, and why. Then do it. - Robert A. Heinlein"

Sounds like big voltage drops. 10G drops 3.3mV per Amp per meter and may be too small for a main feed. Use relays to minimise voltage drops. Measure the voltage drop along feed segments with the respective load on, but also measure the voltage across the load compared to the battery (your grounds/earths may also be suffering drops - especially when all loads are operating).    FYI - a fully charged unloaded 12V lead-acid battery should be 12.7 t0 12.7V (12.67V) assuming it is rested (no surface charge after a recharge; may take 24 hours to dissipate, or several minutes with headlights).