take a look at my diagram before I install

This is how I want to wire my relays (I only show three but I'll have more).
Good/bad/another way?

Also, where can I get 4 pin Bosch relays (SPST)? They have to be smooth on all sides (no raised area for mounting tab) so they fit in my fuse box.

 

The IGN would just be on the positive side of the relay coil. The switch would supply ground only.
The 15A fuses shown were just for illustration. The rating might be different.

 

Your way will work but that’s not the way I would do it. It doesn’t matter if you switch the power or switch the ground. Many of the OEM circuits do switch the ground. The problem is that there’s no good source of ignition switched power in the underhood fuse box. So, with your design, you would be going add a fuse in underdash fuse box, through the firewall to the relays, then back through the firewall to the switches and to ground. The way most people do it would be add a fuse, then to switches, through the firewall to relays, then to ground. Then have the fused constant power coming from the battery. If you choose to have a main circuit breaker or fuse in the constant power to the fuses, that’s up to you. Just make sure the unprotected wire is as short as possible and protected from sharp metal that could cut the insulation and cause a fire.

 

Here is an revised diagram. Three relays are connected to IGN while the other two are connected to BATT. Lets say each accessory/light draws 7.5A, would a 10A fuse be sufficient? If I add a fuse right where I get power from (shown at the top of the page), what value will the fuse need to be? Do I add all the fuses together and come up with needing a 50A fuse?

 

Thanks, but I don't have a stock inverter or the relay box that those wires are in.

The inverter that I'll be installing will be wired by myself.

 

Then you have another choice:

 

Thank you. I was just out under the hood of the truck and I took pictures of those exact plugs.
I'm going to cross reference what I have to the EWD that I have. I'm trying to get all my power from that box, constant 12V and power for the inverter relay won't be an issue, I only needed to find IGN.
I'll get ground either from the battery or the ground point on the fender. The accessories will probably be grounded there. The relays actually won't need ground, each switch will ground pin 85, energizing the relay.

My relay box location. I can't move it closer to the firewall because of the wire harness and I need some space where the harness will exit the boxes.

maybe I can turn the boxes 90*

 

https://www.tacomaworld.com/threads/ign-tap-from-under-inside-the-engine-fuse-box.444373/

 

Found IGN at three terminals, now to trace the wires and see what else is powered by them.

 

I have that but I'm trying to find out where these terminals (circled in red) are getting IGN from. I can use 2B #7 and #8 for IGN. There are no wires in there on my truck but the terminals are powered with the key on IGN only.

 

Thank you. I'll look into that.

 

Looks like #7 and #8 are powered by the 10A EFI No.2 fuse.

I don't have the engine EWD for a 6 cyl.

 

I'd stay away from engine control circuits...

 

I'm thinking the same thing.

Maybe I'll use an empty relay location in that box and make my own IGN. There are terminals in there for "TOWING TAIL Relay" but there isn't a relay in there. Maybe I can re-purpose that?

I'll figure it out.

 

I think I've changed it up a bit. I'll still get BATT 12V from the under hood fuse block but as far as IGN, I'll get that from the driver side junction block in the cab.
I'll send IGN to the switches, then through the firewall to the relays (pin 86). Pin 85 of the relays will be grounded under the hood. I need to probe the drivers side junction block with my multi-meter.

I'll still have the same number of wires passing through the fire wall (one for each switch), they will be sending +12V and not ground.

 

Thanks for posting these details. I am also looking for a way to obtain 12 volt that switches on/off with the ignition. I need it in the truck bed though.

10 - 30 amps is enough.