How much extra power can I draw from the fuseboxes?
 

Pretty much I'm looking at adding an aftermarket amp that'll probably draw around 15A max. My question is: are there main fuses for the 2 fuseboxes (interior instrument panel fusebox and engine bay fusebox)? If so how big are they and how much headroom have I got to play with if I draw supply from one of the spare slots on either of those 2 fuseboxes? If there isn't enough "spare" amperage I'll have to draw directly from the battery but I'd like to avoid that if possible hence why I'm asking.

Thanks in advance :)

You should not do what you are looking to do, as the house wiring can only handle so much as well, for example if you get a 25A amp you probably want to run 8g wiring. Just my opinion. As far as the box having a master fuse, to my knowledge this does not exist.

This x 10000000

Do not skimp out on power. Spend the extra $50-100 bucks for a wiring kit and take the hour or so to run those wires.

If the fusebox has no master fuse to protect its own wiring I wouldn't attempt it. Like you say, wouldn't want to push the stock wiring to far. I just thought there may have been some heads to play with given the number of spare fuse slots.

Run a legit power line from the battery dude. Its super easy and power wire can be purchased super cheap on eBay.

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Yeah, will do. Getting cable isn't an issue I'm a sparky.