Your radio isn't giving you issues, your radio sucks. As much as I despise the garish looks of a new electro-spectacular radio in a subdued 240 dash, you reach a point where you gotta give in to improved performance.

It's been a while since I did a '90, but the neatest, most direct install into a '93 uses a Metra wiring harness kit. If I remember correctly, the '90 also requires an additional harness with a Molex connector for the rear (?) speakers which Metra also offers. These Metra kits are available all over the net at all sorts of price points. (ref: http://www.metraonline.com )

The stock harness remains intact and the amp is bypassed with Metra's harnesses.

The Metra stuff is Volvo-particular, but a little universal and might require some jockeying around of the mass connector terminals. The hardest part is reading the illegible wiring diagrams in the Bentley in order to align the conductors correctly at the connectors.

Since you can splice the Metra harness to the harness supplied with the radio at the bench, it makes the install into the car truly plug-n-play.

I recently changed the factory radio to a new aftermarket in both my wife's 94 model 940 and my 87 model 240.....Christmas presents.
It's still pretty fresh in my mind.
Hopefully I won't confuse the two.

I don't have a schematic for a 1990 240 with the seperate amplifier, but I do have a schematic for a 1991 with the seperate amplifier, and I'd be suprised if they weren't the same.

First of all, In you existing set-up, your speaker wires are fed from the amplifier, and not directly from the radio.

You will need to extend the speaker wires from the ampifier to the radio compartment. The amplifier is inside (center) of the dash. You can find it by following the co-axial cable (not the antenna cable) that connects the existing radio to the amplifier.

The amplifier is a (approx.4") square black box.

Simply snip the wires from the amplifier connector and using new speaker wire, extend each of the wires (use crimp connectors, or solder and cover with shrink tubing) to the radio compartment. There are 8 wires that need to be extended. 2 wires (positive and negative) to each of 4 speakers = 8 wires.

Be sure to mark the wires as to color because this is important when you connect them to the new radio.
Make a bundle of the wires and route the bundle away from any moving parts in the dash (wipers) that may damage them.

The black wire connected to the amplifier is a chassis ground, and the green wire is a (constant) hot wire from fuse #3. There is no reason to cut these two wires, simply leave them connected to the amplifier.
Uplug the co-axial wire that travels from the radio to the amplifier as it will not be used.

At this point, it only a matter of connecting the existing wiring to the new radio.

The speaker wires that you should have in the radio compartment are 4 pairs which include

Left Front Speaker
1- Yellow/White
1- White

Left Rear Speaker
1- Yellow/Brown
1- Green/Brown

Right Front Speaker
1- Yellow/Gray
1- Gray

1- Green/Gray
1- Green/Yellow

These are the wires that you extended over from the amplifier and marked the colors ....correctly I hope.

Also you will have the following wires

1- Green....This is the hot wire from fuse #3. It is constantly hot.

1- Yellow/Black....This is the hot wire from the ingition switch and is hot whenever the switch is turned on. (Acc and Run).
It is powered thru fuse #1.

1- Black....This is the chassis ground.

Blue/ White
Brown........These wires will not be used, just leave them disconnected.
They are for brightening and dimming the interior light system.
Your new radio probably doesn't have this connection.

1- Red.... This is the wire that goes to the power antenna (if equipped).

You will have to pair the wires with the wire colors on the new radio connector.
Simply connect the correct car wire to the correct wire that comes from the new radio.

It easier if you will remove the glove box and the panel that is beside the drivers right knee.

I hope this helps.
Post back with questions.

steve

I just upgraded my 1990 240 with a new Kenwood radio. I bypassed the amp since the radio had 4 channels. I remember I just cut the 4 thin looking speaker wires from the stock Volvo amp. I hooked up 2 at a time to the radio to figure out which was for the rear right and left. I then sodered the wires, wrapped them with electrical tape, and all is good.

No need for the extra amp most all modern radios have 4 channels, you can leave it or remove it if it is in the way.

You will need to identify the speaker wires a switched and un-switched 12 volt source, a ground and maybe an automatic antenna wire if you have one. This can be done with a volt meter and a wiring diagram.
Dan