Sub + Amplifier

[deejay_xb]

Should I install an 50A relay so that the amp will not be always powered? (it has auto on feature based on the cdplayer on/off condition or auto on based on inputs level, but I don't really trust this and prefer that it has power only when contact on)

[Sol]

That sounds like a sensible option deejay.

[deejay_xb]

+battery --> 50A fuse --> relay --> +amp
-battery/car ground --> -amp

the relay coil will be activated by the cdplayer +power cable.

or install the amp in the rear compartment/boot/trunk and use the thick wire (located in left side of boot for trailer) but I need to check when that wire is live, if it's live all the time or only when contact on.. also I don't know how many Amps that cable supports (the amp need max 40A, it has one 40A fuse)

[Sol]

I would be VERY tempted to use a terminal on the battery connection unit, usually there are spare large capacity fuses on there or at least unused positions which are a bolted connection, use wire you know will take the 40A with ease, as pulling more amps over a cable will make it hot, and soft. If the insulation lets go then you have a fire starter, with the only protection being the fuse at the beginning. Best set up is wire that can take 60A, a 50A fuse and then a 40A load.

One of these ones:
   

Use the stereo as mentioned, to operate the 50A relay coil, which has the N/O (normally open) contact pair in series with this wire and on to your amp.

[deejay_xb]

thanks! i will check the battery connection unit to see the spare place in there.
for the Amp in the boot ideea do you think it's bad?

[Sol]

No the amp in the boot itself is not a bad idea, but connecting to any wiring in there if you don't know exactly what current it is rated to supply, could cause you big issues later. I think a 40A supply in the boot, even for a trailer interface, sounds unlikely to exist. That is 480W in power terms.

[deejay_xb]

ok, so the best option is directly from battery with relay, thanks!