I added under seat and dash lights in a 2006 GMC Sierra, but the additional four 5 watt bulbs cause the interior light fuse to blow. I do not want to increase the fuse from 10 to 15 amps since this may cause the wires to overheat. I will get power from a new source, but need to know what I need to connect the lights into the existing circuit and how. I suspect I will need a relay, but unsure how to wire it into the circuit. Any help is appreciated.
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2006 GMC Sierra Duramax Diesel
You have added less than 2 amps of current draw. There is no way that you should be blowing a 10 amp fuse now. There is a problem somewhere. Remove the bulbs from the sockets, leaving the sockets connected, see if it is still blowing fuses. If it still blows the fuse, there is a problem with your wiring.
Easiest way to do something like this would be to use a simple 2 position switch, in line fuse holder and a distribution block. Find a +12v accessory or constant, depending on what you want to do with the set up. If you want the lights to be able to be switched on and off with the car still turned off, you'll need a constant connection. If you want lights only to be on with the key in the ignition turned to ACC (when the radio is on), you'll use an accessory conneciton.
Use an in-line fuse holder with no larger than a 10 amp fuse to the selected connection no more than 6" from the connection, you can find both at the ignition harness of your vehicle. Mount your switch where you want, and you'll run the +12v input to one end of the switch, the output side of the switch to the input side of the distribution block. Then it's as simple as wiring your lights up to the block.
By the way, don't put the fuse in the holder till you've made all your connections. Just a safety procaution.