I do like a bit of bass when driving, and have seen a slimline sub from alpine that looks like it could do the trick of filling out the low end, but keeping the boot free for bikes. I think it’s a case if taking a feed from left and right speakers, and a power feed from say the back of a cigaret lighter socket and jam it somewhere out of the way.
Anyone have any experience and opinions?
Before anyone says why bother, it’s because I’m used to the bose system I had for a while in the (I know) Audi which comes with the built in sub which i loved, and moving to a Golf GT in 3 weeks which just has the standard 8 speaker system.

I used to have a slimline 10″ sub in my old car (MR2) purely because nothing bigger would fit. I built the box myself, using a sub-enclosure calculator to work out the dimensions. Sounded nice and punchy without any drastic loud spots at random frequencies, although I put that down to luck as much as I do to my woodworking skills.

Dunno about the wiring you’ve suggested though. To power the amp, mine was 12v +ve 8-gauge wire direct from battery, normal size wire for switched power spliced from the back of the head unit and an 8-gauge earth wire fixed to a bolt on the passenger seat subframe. Decent phono cable from the head unit to the amp. That was for a tiny 100W amplifier, so if you wanted louder then maybe plump for 4-gauge power and earth wire. And speaker wire does make a difference when you’re connecting the sub to the amp. I used some cheapo-but-excellent £2.99/metre stuff from Richer Sounds. Miles better than the bog-standard speaker wire you get from Halfords.

A system such as the one you propose will require some heavy cables as you only have 12v in the car and most good subs will be about 200w the current may blow your fuse.
Your best and safest bet would be to run a cable front to back from the battery to the boot, a lot o hassle but worth it for the inevitable future upgrades once you start going down that road.
I have picked up some good stuff from the car audio direct web site for my van.

In my experience with guitar cabinets, smaller enclosures make for a thinner/weaker sound with less low end. Bigger cabs with heavier and thicker wood give a bigger sound.

If i were you, I’d consider a powered sub. I have a Sound Ordnance b-8pt, and whilst it’s not earth-shattering, it’s plenty to fill out the sound and will still shake the mirrors a little. If I really had to, it would be easily removed also…

If its the sub i think your looking at it has a built in amp. As has been said give it its own power feed fused at the battery cigaret lighter will just blow its fuse when you turn the volume up. For the audio input i think those subs can use either a line level input or speaker output in. If your stereo has line outs use them for better sound quality. A new boxed one should come with instructions for wiring and setting up.

Many moons ago I had an Infinity Basslink. Not especially slim but pretty compact for a combined amp and sub. All the cabling instructions came with it. You can get a basic amp wiring kit from Halfords for not much money and it’ll include the big fuse and battery cable.