Where are you mounting extra relays for wideband O2 & other aftermarket components?

When adding additional components like a wide-band O2 sensor/controller to a stock+ car, where do you guys put the additional relays?

I'm modifying a stock car (Subaru Impreza GC8 STI Type-RA MY97 V3) with an aftermarket ECU. There'll be a few aftermarket components needing relays (WB O2 controller, electric power steering motor, electronic throttle). Do I add an additional fuse box?

What do you guys do?

Depends on what the relays are driving, where the power source is, and where I need to connect to.

Electric fans, I often put the relays in the engine bay next to the fan(s).

Fuel pump relays can be in the front next to the battery, under the dash or in the back, depending on the system that is controlling it, where the battery is, etc.

Other relays could follow similar placements, things like WBO2 controller power relays will usually be under the dash, or in the engine bay, again depending on what is easiest or best.

Sometimes I use full fuse/relay boxes, especially when I'm installing a bit more complex system in an older vehicle, when I say "complex" I mean one that is usually involving multiport, and controlling multiple devices that I would like to see individual protection or control for. As an example, a 280ZX that I installed a Megasquirt MS3 on, I used a fuse box that also had relays to fuse and control everything the fuse box was next to the battery.

On most Holley Snipers I install, I use inline fuse holders next to the battery, because these are usually only two to four fuses needed depending on how much the Sniper is controlling and what else is installed, I also like to modify the harness and put the ECU and Fuel pump on their own fuses, instead of the single fuse that Holley uses, with relays elsewhere, or also next to the battery. In a Chevelle I'm completing, I have 4 inline fuses off the battery, MSD, ECU, fuel pump and cooling fan, and attached to the rad support hidden by the battery on a bracket I have 3 relays that control the fan low, fan high and fuel pump.

So, in the end there's no hard fast rule about where fuse boxes and or relays will placed, other than there will be a fuse at the battery to protect each circuit, or where the power is tapped into elsewhere in the vehicle as should be. Relays can be literally anywhere, where they make the most sense or fit best.

What are these fuses and relays that you talk about?

For the cost of a PDM, I would install one over adding a bunch of fuses and relays. When I had my shop and did wiring, the cost of doing a rewire with traditional fuses and relays compared to using a MoTeC PDM (essentially all that was available at the time) was comparable, especially once the time involved in wiring all of the fuses and relays was taken into account.

I would recommend replacing the loom in the car, rather than adding patches on to the existing loom to accommodate the DBW, EPS, Lambda controller and any other components, it may work well now, but experience with old Subaru looms is that when you start fiddling with them, they develop issues.

Thanks for the insights, guys!

@Six_Shooter Appreciate the breakdown on logical placement. Keeping the relays under the dash definitely makes the runs cleaner and shorter. Do you have a power block that you branch off from, or do you just run splices from a terminal that's placed on the battery terminal itself?

@BlackRex Haha, fair point on the PDM! You're right about aging 90s Subaru harnesses getting brittle once you start tapping into them. I'm definitely leaning away from hacking up the loom.

A PDM like the ECUMaster PMU-24 is on my radar, but in the meantime, I'm weighing whether to

- consolidate the extra circuits into a clean auxiliary fuse/relay module under the dash,

- commit fully to a fresh chassis/engine sub-harness setup with installing a MicroPDM (28 outputs from 3 units),

- or run the relays/fuses like @Six_Shooter suggested to get things up and running quickly

To give a bit more context on what I'm trying to achieve

My plan has two phases. Currently, the factory EJ20K heads need some bench work, so in the interim, I'm dropping in a complete EJ20Y long block (dual AVCS) to get the car running. Once the EJ20K heads are sorted, they’ll go onto the EJ20Y block for the final build.

This means I initially need to integrate the EJ20Y harness into the GC8 chassis, but eventually, I'll be refreshing/rebuilding the EJ20K harness (and likely moving to a PDM setup) when it's time to put on the EJ20K head.

It's a far-fetched thought but I was wondering if it is possible to avoid doing the wiring work twice so that I don't end up building a temporary harness that I have to completely scrap later.

For those with experience organizing a build like this, what's the most efficient path forward?

I was thinking of going the MicroPDM route.

2026-07-24: Edited to improve clarity