A simple wiring modification will allow you to power the radio, interior lights and 12V socket from the auxiliary battery in your 1986-1991 Vanagon.
On 1986-1991 Vanagon's, fuse #3 protects the radio, interior lights and 12V socket. By re-wiring the power supply to fuse #3 you can power all of these accessories with a single fused wire from your auxiliary battery.
There are several ways to run the wire from the auxiliary battery to the front of the van. The best route will depend on the location of your auxiliary battery as well as your wiring proficiency. The wire you choose should be at least 10AWG-12AWG and will need to be protected by a properly sized inline fuse or circuit breaker (15A usually does the trick).
The most common location for a small auxiliary battery is under the drivers seat. In this case, the best installation method is to drill a hole in the battery box, install a rubber grommet and lead the wire under the van and up through one of the factory installed grommets in the front. If you choose this method, be sure to protect the wire with split loom to avoid chafing or otherwise damaging the wire. Use cable ties to support the wire by attaching it to non-moving points under the van.
People who wish to avoid drilling holes in their van often run the power wire through the opening in the aisle side of the seat pedestal where the seat belt warning circuit wires attach to the seat belt receiver. When choosing this route, be sure that there is a rubber grommet installed in the access hole to avoid damaging the wire. Guide the wire through the grommet, then under the carpet up to the front being careful to keep it away from sharp or moving parts in the emergency brake or the shifter mechanism. You should be sure to protect the wire with expanding mesh or vinyl sleeve if you use the walkthru between the seats on a regular basis to avoid damaging the insulation and causing a short to ground. Even a couple layers of duct tape will help to protect it if that is all you have on hand.
Once the wire has been pulled to the front of the van, you have two basic options to connect it to power the accessories on fuse #3. Whichever method you choose, the first step is to remove fuse #3 permanently, since you no longer want the accessories to be connected to the starting battery.
The simplest method is to install a 1/4" male disconnect terminal onto the end of the wire and insert it into the portion of the fuse that does not have power present from the starting battery. The factory wiring changes mid-way through production and needs to be confirmed with a test lamp or multi-meter before making your final connection. Remember that you will be inserting the 1/4" terminal into the side of the fuse that does not have power present from the starting battery.
A slightly more involved, but significantly more secure method to attach the new power wire is to connect it to the positive wire going to the 12V socket (lighter socket) on the dash. This can be easily accomplished with a 12-10GA Posi-Tap, which will allow you to easily remove this connection if you ever decide to sell your van and want to return it to stock wiring. Be sure to use the correct size Posi-Tap to minimize damage to the wire you are tapping into.
To build in some future expansion possibilities to the auxiliary power wire hookup, many people install a small auxiliary fuse block that will allow easy connection of a few extra circuits for the front of the van with very little trouble. Use a wire rated to carry the total load you wish to run (10AWG is the absolute minimum, 8AWG would be better) and size your circuit protection to match.