wherever you put the relays allways gets wins and loose factors...

-inside the cab, save from heat, dust, grease or corrosion and also gets a clean engine bay view but is true you loose some on distance but is not that critical.

about terminals... bulkhead terminals are able to drive the halogen beams power from relays. IF YOU THINk relays and plugs ALREADY use those spade terminals, why not the bulkhead if they are the same kind ?... just need to take care about the positive source coming from alt/batt... I use the alt source to keep the right reading on ammeter. ( yes I keep the ammeter )

-On engine bay, of course opposite pros and cons than above

I didn't cut wires, just removed the terminals from original plugs cavities and moved to relay plug, then run the same color and gauge wire from relay to original plug cavity to get everything back to normal.

Darren, relays are just to headlights, however you can use a realy for anything on your car, including parking lights.

I wouldn't use them for turning/hazzards. If you use a relay on turning/hazzards, flasher won't get same heat than normally gets with all bulbs load and will get a VERY slow blinker function ( if any blink ) because bimetallic on flasher never will heat. Just like when you don't have bulbs around ( or damaged ) and just cluster turn indicators... they never makes blink the flasher by themself, not enough load