Wirering diagrams for the 155 is hard to get(Juan has , but wont let go of it), but you can do without, here goes:
You need something to measure with, the best is a voltmeter, but a 12V bulb with two wires can do the job.
The gruond is black, from that you measure on the other wires until you find permant power (i think it is red, not shure !), then switch on the ignition and do the same to find ignition power.
Connect the radio and switch it on, then find the speakers by connecting one speaker at the time to the radio (the speaker wires are in pars the same colour, with or without a stripe).
If your car is equipped with antenna build in the rear window, then the antenna-amplifier needs power (+12V) the wire is red & green.
I hope this helps.
#196 - 2,5 V6