A few ideas:

-if the wire guage is the same, then so long as no other device is being powered by that cable, that should be OK. If the amp is being powered from the same 16 gauge wire, then it is unlikely to be sufficient. However, I would say that you should be more concerned with the earth wire gauge - make it as big as possible for both the empeg and the amp. The ground should only go to one point, near the amp. If it gives you any idea, I have used rigid single core feed wire for power and ground, the same stuff used to install electric cookers.....

- the voltage test is worthwhile. Twist your installer's arm and get him to do it. If he refuses, ask for your money back

- "bonnet" is the hood (engine bay cover). The electrical contact between the bodywork and the hood is frequently poor. If you connect the two permanently with a battery strap (braided=woven) then the bonnet lid acts as a shield against radiated noise from the ignition and charging systems. I have had good results on two cars doing this (not tried it on a Beemer though).

- you should be able to ask your dealer what sort of plug leads are on your car; if they are Bosch, they will state clearly on the lead "Radio Supressed" or carry the letter R after the lead number.

- you should really have the sled earthed to the car body, both for shielding and safety. Grounding the shield of the signal leads to the DC earth of the car is asking for it, I would have thought (earth loops, ground bounce spikes, horrible stuff like that).

- ask your BMW dealer about a supression kit, I know they used to sell them seperately.