There are pin-outs for the 6-pin din amp cable listed here on the board, to allow you to make or adapt a connector with the din plug at the radio and RCA jacks for the aftermarket amp.
If you plan to run speaker-level signals into another amp, make sure you have an aftermarket amp which will accept speaker-level input--otherwise, the first and last sound you hear from your new amp will be the same frying sound.

Are you looking at installing an aftermarket HU with its own amplification and not needing a separate amp? If your Volvo unit is presently running the same number of channels off the HU (not to be confused with the number of speakers--see later info), then you can get an adapter from any of the big retailers--Scosche, Metra, Crutchfield, etc.--which will plug directly into the factory harness and wire up to the connector on your aftermarket unit. You will need to find your amp either way--the volvo wiring diagrams will often give you at least a general area to look in.
Some of the Volvo units run two channels off the HU (usually the dashboard speakers) and four off the amp. This can lead to rewiring and possible speaker changes if you are going with only 4 channels from the aftermarket unit.

Speakers vs. channels: The standard for automotive stereo is pretty much 4 ohms per channel. The aftermarket speakers are usually 4 ohms, which means 1 speaker per channel,= 4 sspeakers for 4 channels. One way to get 6 or 8 speakers on a 4-channel stereo is to have some of the channels run pairs of 8-ohm speakers in parallel (which, by magic somebody else can try to explain, gives the same effect on the amplifier as a single 4-ohm speaker). E.g., if your car came from the dealer with pairs of speakers on the dash, front doors, rear doors, and rear deck, you probably have 1 pair of channels for the dash, 1 pair for 4-ohm speakers in the front doors, and 1 pair of channels running 8=ohm speakers in parallel on the rear doors and rear deck. Your wiring diagrams will usually tell you!If you wwant all 4-ohm speakers, you need 8 channels, or component speakers splitting the frequencies with crossovers.

For about $25 you can get a year's subscription to ALLDATA DIY for your specific car, including the right to download and print the wiring diagrams. It's worth it, even if that's the only time you ever use it.

What model radio do you have now? Use the model number as a search request on the bboard, and you might find a lot of info to save you problems.

Good luck!
PS: Some of the above info may not be very clearly stated, or may even be skewed--I'm not an expert. The info is all here on the bboard, from people who are experts.

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We have met the enemy and they is us. [Pogo] '99 S70 cop car : Rough Rider tires& suspension, Walmart fog lights, speakers by ebay, ambiance by Pall Mall, trim by Le Duc d'Tape