The current draw of guage lamps, LEDs, etc. will be stuff all and can be spliced into the existing harness.
For stereo, work out the potential maximum current draw of the system (ie. sum of all amplifier fuses) then run adequate cable to a dedicated distribution block/harness for the system.
Neons could be wired through switches from the stereo harness too. Be sure to include the current draw of all the neons in your calcs when you spec the cable guage for the run to the distribution block.