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I’m installing a Retro Sound head unit.

I’d like to use the slot labeled one or slot labeled 2 to connect the 12v constant wire. I tested slot 1 with a MM and its a for sure constant connection.

I would also like to tap off the wire that’s connected to the fuse block slot labeled “Radio” for the ACC switch as well.

Any reason why I shouldn’t do it this way?

Only concern with #2 is that you may leave the radio on and run the battery down

If the HU requires a connection that's alive all the time, you could do it that way. Seems unlikely. As mentioned above, if that's the usual power lead, keeping the HU live all the time will run down your battery. Sometimes these devices need a trickle of power to keep memory alive; however, that's usually supplied by its own battery or super-cap.

Also, if it's a whomping big head unit, those connections may not be stout enough to supply the total power needs.

In the USA, a radio is usually connected so that it requires the key on or in accessory position. That's the main intent of the accessory position on the ignition switch: so you can send power to the radio without powering the ignition.

The manual on the Tom Collins site can be hard to read, but my printed copy from BJ's is better.



The panel printing is inverted from what you show in your picture. You see at the bottom there is a connection marked "RADIO" that would be ideal, I 'd think.

I don’t believe this HU is that powerful. It’s a built-in 300-watt (peak power) Mosfet amplifier. 25 watts RMS x 4 channels

Here’s some pics to clear things up regarding the 12v constant (yellow wire) and key on Ignition ACC (red wire).

I believe the radio turns off when the ignition is off because the Red ignition wire connected to the “Radio” slot on the fuse block loses power since the key is out. So even though the Yellow Constant 12v is always live, if the key is not in, the radio should stay off thus unable to drain the battery.

Here is the wire harness




Here’s what I’m thinking about doing:

Seems pretty clear. The radio connection likely powers the tuner, and the constant for the amp? The yellow wire looks thicker, which suggests that. I don't understand why it would need constant 12V. Maybe it's the other way around, and the constant power is maintaining the display settings. If so, that only needs a few microamps. You could ask the manufacturer.

25 watts RMS at 12 volts is about 2 amps. You can look at your speaker impedance. At 8 ohms, 25 watts is about 2 amps per channel at 100% efficiency. P=I^2R. It's not 100% efficient, so I'd guess the current requirements are quite a lot higher. 15 amps may be ok, 3.7 amps per channel. Your fuse should be something like half the peak current demand.

I'd say hook it up that way, and see if it pops the fuse. I expect it will be ok. What size is the wire? 14 ga is roughly something like 15 amps. If it pops the fuse, make a new feed that goes direct to the starter solenoid via a 20A circuit breaker close to the solenoid.

To play it safe and to only do this once. I’ll connect the yellow 12v constant wire at the solenoid and add a breaker.

I’ll connect the the thinner red ACC wire to the fuse block slot that says ‘radio’ and see how it goes.

The speakers are each 4 ohms.