12v battery point

[NMcC]

Anybody ever installed an aftermarket one into a 325i Sport? Any handy ways of putting one in?

I have a 12v ciggarette point already but the cable to it has taken to a gearstick gator for power, just looking one to charge phones/satnav etc.

[Brianmoooore]

I doubt that you've used the power feed to the cigarette lighter to illuminate a gear knob, since this is permanently live, and would result in the knob being lit all the time. The OE wiring for a gear knob uses a splice into the illumination wires for the cigarette lighter.
If you want another 12 volt permanently live socket, it's quite in order to splice that to the red/yellow wire to the lighter, or to take it from the accessories socket behind the glove box.
If you want a socket that's switched with the ignition switch, then, assuming you don't have heated seats, you can take it from the white/brown wire at the accessories socket, which is fed from fuse 16, and suitable for supplying up to 15A.
Note that this wire is missing on some lower spec. E30s. You can see if it's fitted by seeing if there's a bucket fitted into the outer slot for fuse 16.

[NMcC]

Bought a new 12v point today so ready to tackle this at the weekend,

How do i take it from the accessories socket? how do i make a connection?

[Brianmoooore]

Either find/modify a plug to fit pin of the socket, or remove the socket from its mount, turn it around, and splice to the wire behind it.
Take the negative for your socket from the earthing stud behind the RH side of the glovebox, where all the brown wires go.