That kind of load would really be best in a cool space and closer to the loads. That relay box you saw in the LR is a common Ford part, there are similar boxes in the dash, under the air cleaner, and often along the fenders. The best part is they all take interchangeable Ford relays, there are two sizes, equally reliable. I mixed the parts from some spare wiring to make one for my extra circuits, I have a four relay combination in it with a flasher cut and epoxied into it. I mounted that under my master cylinder, with two zip ties. They all have covers to protect them some, if you use them make sure to place them so any water can get out and not be trapped in the relays.
An important feature you get with remote relays and switches is that the switch wires are very low power, and cause little trouble with burning wires, connections etc. The important wiring critical for amperage and great connections are all at the relays and to/from the battery and loads. That's safer for where you place the switches, and those connections can be very quick small wired crimps.
FYI, a 94+ Mustang has a nice main PDB(power distribution box) under the hood. They are easy to come by etc. You can put together lots of combinations.
Relays are somewhat simple. There are commonly four terminals, two trigger wires and two load wires, and usually either of either pair can be ground or power. The fifth terminal that some have is a normally closed point to the #30 main power terminal. It's usually used to turn off a circuit, feed power or complete a circuit without applying power, until it needs to be broken. That doesn't have to be used, it's just another choice.
Here's my first mail vehicle wiring, a 95 Crown Vic that I changed my mind about later. I like to use the factory stuff when I can. Look at the first picture, and guess which PDB is stock?
The stock one has holes in it from switches that I replaced with the James Bond Mod.
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