keep radio on

Ok here is what I want to achieve. I want to be able to when I park and when I turn the car from run to accessory, not have the radio cut off on me. As it is right now, when I turn the key no matter how fast I do it. What I don't want to do is build a RAP-like module. I was thinking along the lines of using a capacitor and diode. Something like this: Does everything look ok with that?2013 Chevy Impala LTZ

Better off using a diode from each of Acc & IGN that feed a relay (#86; with #85 to ground, and maybe a spike suppressing diode with Kathode (line end) to #86 and other end to #85). The relay (#30) should be feed from the battery via a fuse with output (#86) to the Radio's power (Switched). The supply to #30 can be your own new/dedicated cable(s) with fuse from the battery +12V (and battery -ve) as many (of us) do, else an existing constant +12V circuit with adequate supply. Though your circuit is good for smaller and less voltage sensitive stuff, the diode/capacitor introduces a 0.6V diode drop and both the diode and cap have to be BIG to supply the required Radio current and supply time. Plus the gradual voltage drop and cause problems (it's like a brownout situation - an unclean OFF). Using the diode/relay method means low(ish) power diodes can be used (eg, 1N4004 else 1N4007 1A 400V else 1000V diodes for all cases - ie, need 3; get 5 - only ~ 20c each). The relay can be any 12v SPST (else SPDT) relay that handles the Radio current - eg, common 30A auto relays.

Plus one for using smaller value components and a relay to supply the load. the12volt • Support the12volt.com

Is it an aftermarket radio?  If so, the red wire of the unit draws so little current that a 470 microfarad capacitor should do the trick. 

I was assuming the switched was the main power. But that isn't always so - it cam be merely a signal wire, though sometimes the constant is only for memory power.

Back in the day, that was how it was. Everything built post about 1985 uses constant to power everything in the unit. Hence the larger (10 amp) fuse on the constant wire. Notice the Pioneer units with the resistor in the little black tubular thingy in the red wire. Wow, I typed thigy and the spell check notified me. It took thingy. Wow