running an Ign wire?

im running a fuse block in the back of a car im workin on
im running 3 screens,reverse camera,video amp,power inverter,neons,etc.. and want it to come on with the ign.
now i figured id run a 10ga cable off the ign wire at the key (2000 BMW 328ci)and run it to the fuse block. now should i run a relay to that the to the block or run relays after the black to each component???
also how do i conect the 10ga to the wire itself will it be of to solder that thick wire to a smaller one??
or is there a better way to do what i want
thanks

Run a small primary wire (red) from the main ignition wire in the ignition harness up front. Fuse this line with a 3 amp fuse close to the connection in the main harness. Next attach this line to your relay in the back of the vehicle. The relay will be used with a main power wire attached to the battery. One side of the relay is going to be attached to the battery, the other side is going to be going to your fuse distro block.  Here is the skinny on what you want the relay to do.

Connecting Additional Devices to the Remote Turn On Lead

Using a 30 amp SPDT relay, connect terminal #87 to constant 12 volts positive with a fuse rated to the sum of the additional accessories you've added and the components you need to turn on. (If you have two fans rated at 5 amps each and a neon light rated at 10 amps, you would use a 20 amp fuse plus 200 ma for each amplifier and processor.) Connect terminal #85 to ground, terminal #86 to the remote turn on lead from the head unit, and terminal #30 to each accessory with an appropriate fuse. A fuse (not shown) could also be used between the output of the relay (#30) and the remote turn on lead of the amplifiers and/or processors for extra precaution. Pin 86 would be your new fused ignition wire coming from the front. Pin 87 is +12v from the battery. When you turn on the ignition, this in turn now engages the relay and pin 30 becomes a +12v ignition line.

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Top Secret, I can tell you but then my wife will kill me.

its ok to run all the accessories off of one relay its not going to draw to much???i was going to run a realyoff of each fuse.
also how would i conect the 10 gauge cable(is that thick enough or to thick??) to the ign wire just solder and tape it ok?
thanks

Most Bosch type relays are going to be good up to 30 amps. The relay should have this stamped on it for that matter. You are not running a 10 guage line to the ignition wire at all. You are running a small primary wire to the main ignition line. This line (fused at the ignition) then turns on the relay at the back of the car where your equipment is going to be. Look closer at the diagram.

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Top Secret, I can tell you but then my wife will kill me.

ok i understand now
so instead of 30 going to each accessory it will go to the fuse block input and then each fused terminal will go to what i want it to go..
so instead of running 10 gauge run like a 16ga wire and them maybe use the 10-14 gauge or so for the constant 12v to the relay
thanks