HELP! where to ground my amp>S40

For 600WRMS output from an amp, you are going to need to supply it with 1000W plus in DC, i.e. upwards of 80A minimum. Thats close to the total output of your alternator, which is supposed to run all of the electrics in the car and recharge the battery as well. At that power level, you need the earth lead to be as short and heavy as possible, and you don't look for a place to mount it. Unless you can find a stud or nut literally within four inches of the the earth terminal on the amp, you create one, by drilling a hole about 1/2 dia through the body, strip an area about 11/2 in diameter back to bare metal on both sides, and get some nice brass fittings, 1/2in x 1 inch bolt, two 1/2in brass nuts (full, not skinny locknuts) and a couple of 11/2 x 1/2 x 1/16 brass washers. Make a sandwich as follows; bolt head, washer, bare metal body section, washer, nut, earth cable lug, nut. As suggested, the reason that the earth has to be ultra short is that it will create a "feedback loop" if it is longer, as the earth currents will create a significant voltage drop which will modulate the input signal acting as a drive source for the amp. Also seriously consider the following upgrades; Second alternator, because 80% or more of the originals capacity will go feeding the amp, so the engine electronics, lights wipers and other roadworthy essentials will suffer from lack of power. Second battery to act as a buffer for the amp, located within inches of it. This ensures that the amp gets all the power it needs for those big booms, and isn't held back by the cable feeding power to it. A 30AH gell cell should be adequate, but mount it securely and ensure that the terminals are well insulated. A good set of ear plugs and a long range remote control, so that you don't go deaf in the first hour of testing this sucker.

cheers dude, unfortunatly i dont have the confidence to do what your asking me to do, dont wana drill holes in my car maself. am gonna try that that big screw thats currently sticking out, if it dont work i might get it earthed by a car audio place. incidently, what does this statement mean in english? "As suggested, the reason that the earth has to be ultra short is that it will create a "feedback loop" if it is longer, as the earth currents will create a significant voltage drop which will modulate the input signal acting as a drive source for the amp." thanks also, any idea where to buy a cage for a kenwood headunit?

thanks for the replies people. last question, ive had another look at the boot, now what about the metalic panel underneath the rear shelf? as in when you stick your head in the boot and look up at the parcel shelf where the speakers are mounted, theres lots of holes on the thin metalic looking panel, can i use this pls? thanks again

you should be able to use that to earth your amp im running a 400 watt one in my 940 boot and its earthed with one of the securing bolts