Sub, amp and wiring questions

[Bugster_MR2]

Hi

I recently stumbled upon a barely used UD sub enclosure with JL audio 8 inch speaker for a decent price. I want to install it in my mr2 with a Rockford Fosgate PBr300x4 4ch amp. This unit is very small and makes sense in our small cars. Two years ago I installed Polk Audio DXi6500 (IIRC) component speakers in the doors. I also have a good android HU. I am disappointed with the sound of the Polk speakers. I believe they need more juice to play properly. I want the PBR300x4 to power everything.

What is the best way to connect the door speakers to the amp? I presume pulling new cables from the amp and through the door grommets is hard and dangerous to the existing wiring.

Is it okay to connect the amp to the existing wires behind the HU, after cutting them off from the HU?

[s12vea]

I always tap into the wires from the head unit ISO for the speaker outputs. Keeps it nice and simple and easily reversible.
The rears bins or behind / under the seat is a good place for amp also

[RubixRonnie]

My amp is stored in the rear bins and strapped down. Best place for it.
I also tapped into the head unit and ran the wires under the sills as the plastics are really easy to take off.