key off system power

I have a 1980 VW Vanagon, It doesnt have a acc. position on the ignition switch and when I turn on it starts the fuel pump. I have an idea of how to wire a separate switch that will power the head unit from my second battery without back feeding the vehicle systems and just wanted to run it by the experts first. My plan is to put in a SPST switch connected to a standard automotive SPDT relay. connect the switched power lead from the head unit to the common on the relay (30) then connect the ignition switch power to the NC connection (87a) and power from the alternate battery to the no connection (87) so when the ignition switch is off I can turn on my alternate power switch and the HU will get power from the second battery. when the alt power switch is off but the ignition is on the HU will get power from the vehicle harness. of course everything will be fused as needed. the rest of my system already gets power from the second battery. the reason I want to do this is because we still have a couple operational drive in theaters here and they broadcast the sound over FM radio. I figure this way I can have the radio playing but not have to power the rest of the vehicle systems and the fuel pump

Why not just have a SPDT switch select the power source for the HU - ie, via IGN or from the battery? (That's what I used to do until I standardised on direct from battery.) But I am curious as to how you have you second battery connected.... Usually if the 2nd is for the audio system, then the 2nd batt should only be connected when charging or when needed and NOT permanently. (And it can power the audio as desired, and should utilise a low voltage cutout aka battery-protector.)

the main reason I was going with the relay is I didn't want the center off position of a standard SPDT switch. The ones with just on-on are hard to find locally around here. My second battery is wired to charge when the ignition is on through a high current constant duty solenoid and is therefore disconnected from the vehicle electrical system when the key is off I know I could just wire it up to turn on and off with a switch from the second battery but my luck I would forget to turn it off when I got out of the vehicle if the volume was turned down.

Two position SPDT switches are more common than 3 position. I'd have another look if I were you. But yeah - nothing stopping you using an SPST switch to energise an SDPT relay for to get the power (from any source). You could even include a low voltage cutout (aka Battery Protector - like the MW728 ~$20) to disconnect the HU if you forget to turn it of, although in that case, maybe a self-latching circuit is better - a push-button to turn on, and another to turn off - similar to the following.... (Though this diagram includes the voltage/charge sensing inter-battery or load connection... The latching part is the Push-On & Push-Off RHS of the diagram. An MW728 output is used to supply the +12V fr the Push-Off leg.)