Any suggestions on optimum/easy placement of this relay?

I mount the starter relay to the lower trunk/engine bay firewall, within a few inches of the starter. A short path for the main 12V in, from starter lug, and back to solenoid.

In the trash can

I have installed dozens of these and don't know of single failure.
I often zip tie them to the starter solenoid.

Jim

That's because you live in California.

I seen a few fail, I have a brand new one here that failed, I'll see if it works for a horn.

Yes I am in California
And one of the reasons is the weather

Jim

Yep it's winter here....I think I'll water the lawn just for fun.

Thinking about building something like this in the backyard this summer, I have lots of water and it's free

....But that water kills bosch relays

I use Ford. Haven't had to Fix Or Repair Daily.

Can you imagine how badly people be losing their shit if it was a Fiat relay?

The engine compartment is usually protected from the elements, moreso than under the chassis.

That said, remove and route the existing control wire (Yellow - from ign. switch) at the starter solenoid up to your relay in the engine compartment.

For a new source wire to energize the starter solenoid, find (a) an ign.-activated 12V circuit in eng. fuse panel to tap into for the power circuit. Route this new power wire from relay to the existing starter solenoid. Or (b). The battery cable at the starter solenoid is also a handy source for 12V. If used, install an easy-to-access inline fuse since it's always "hot".

Here's a diagram from which you can revise for your circuit. I designed it to use a remote momentary switch to crank the engine (saving the NLA 911 ign. switch) and to provide full battery voltage to energize the starter solenoid. Click to view attachment
As described above (b), this circuit uses the existing battery cable to source 12 volt power for the new relay.
Click to view attachment

Sherwood

Engine compartment, someplace where you can get to it easily. I have the one in my street car mounted underneath near the starter; late last year I had a no-start that ended up being a blown fuse. Of course, I was in nice work clothes so I had to bump-start it.

The one in the race car got moved to the engine compartment, and I'll be moving the street car one come nice weather.

I've installed literally dozens of these and never had one fail. If you mount them under the car, you run that risk. I mount them in the engine compartment and run the wires through a firewall grommet. Takes longer, but they last and work great.