There are two ways to tackle this.

1) Hook into the manufacturer's official "access points".  These normally
only cater for car light repeating and NOT additional electrics
2) Use 7 way bypass relay with additional electrics

Method 2) is what 99.9% of tow bar fitters in the UK will use.

I've used both methods.  1) with a Tempra 1.8ie where I only needed lighting
control (sidelights, brake, indicators, fog & reverse)
Method 2) on a Tipo, Stilo and Croma (2005)

Method two requires

1) One, preferably two, power feeds directly from battery and each fused
(single 30Amp or twin 15A)  If using single 30A feed then this has to be
split into two separate fused 15A supplies in the rear end of the car.

2) 7 way bypass relay unit fed from single fuse 15A supply

3) Voltage sensitive power relay fed from single 15A supply

4) Item two is fairly easy to wire and requires one to spur off the existing
car wiring.  The bypass relays only take a few milliamps from the cars
circuits and are thus compatible with the onboard bulb failure systems.

5) The auxiliary electrics are also easy to wire.

One point to note is DO NOT overload the existing car wiring.  You MUST use
relay switch circuits fed DIRECTLY from the battery.  This requires a
single/twin power feed directly from the battery to the rear of the car.
This is a MUST!

I can look up the wiring colours for your car but would also suggest that
you can trace these by inspection and use of a simple volt meter.  You are
looking to identify:

1) brake light wire
2) side light wire
3) reversing light wire
4) rear fog light wire
5) left rear indicator
6) rigt rear indicator

The rest is a matter of common sense :-)

Nick