If it looks like this, it's an SC-816.
Pressing and holding preset number 6 when you turn the stereo on will allow the stereo to function for about 3 minutes, before it demands a code again.
To get the code if you don't have the VIN of the origin car, you have 2 options -
[1] Send it away to a shop to retrieve the radio code. There's a place in the UK that does this; it's not cheap.
[2] Brute force the code from the radio, by trial and error.
I picked up a couple of these without codes for peanuts. Then to brute force the code, I set them up on a benchtop power supply, and then printed out a 'code sheet' from 1111 to 6666. Since I'm a Volvo Tragic with no life and a lot of spare time, I kept putting numbers into the thing until I hit paydirt, crossing them off the sheet as I went - enter 3 sets of four numbers every 2 hours or so - the display says OFF after 3 strikes, so then wait till it displays CODE again; rinse and repeat, until a frequency is seen. The first one came to life in a day or 2 (first column of the code sheet!) but the second one took a few weeks to crack. The radio must be left switched on between code attempts - ie displaying OFF - or otherwise you have to wait 2 more hours from power-up till your next code attempt.
You can't download a 'code cracker' for these radios; all of them are scams.
If you want to read the code from the radio itself, it means reading the memory chip from inside the unit. It's a little surface-mounted bastard - a 3132 - which IIRC is an 8-pin job jammed in a real PITA location on the main PCB. Which may explain why code extraction via a shop isn't cheap.
Oh.... and wiring, I hear you ask?