Please Help with Switch/Diode Question

Well I've got a question for anyone with some electronics expertise.  I purchased a PAC AAI-GM12 auxilary input for my truck.  I don't like the toggle switch that came with it and I'd like to replace it with a lower profile rocker switch.  The problem is that I don't have a DPDT on-off-on rocker switch to replace it with, but I do have a SPDT on-off-on switch I'd like to use.  I thought that I might beable to accomplish this with a diode, but have no real experience with them.   Here is what the original switch looks like:

The thin black line is a jumper that jumps those 3 poles on the switch together.  From what I can tell when the switch is flipped in one position it connects all 3 together Black-Red-Grey.  When flipped in the other position it just connects the Red and Black.  I thought I could accomplish this on a single pole switch with a diode like this:

Any ideas if this will work?  If so what value of diode would I need? Thanks-Rick

Hi Rick, if the red wire is 12V+, then yes, the black wire will have 12V+ in either position. The value needed depends on the amount of current draw that exist on the black wire.

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Thanks for the help.  I bit the bullet and ordered a DPDT switch-just like the one that I replaced except in a rocker style.  With the internet it ended up being pretty easy to find one.  BTW Mouser doesn't have a $25 minimum on parts orders for anybody looking to buy a couple of parts for a project.