There is no 'screw'. The image you added is not applicable to this process.
This thread already talks in detail about how to lower the package tray, but I'll go over it again.
My 1990 Factory Service Manual says this:
Control Module/Relays
NOTE: The control module and relays are located on package tray in luggage compartment.
Removal
1. Ensure that ignition switch is in the OFF position.
2. Locate pushpin on left side of tray. Disengage pushpin.
3. Swing tray down using slight force.
Installation
1. Place module or relay securely into retaining clips on tray.
2. Connect wiring connector.
3. Swing tray up, locate slot with metal clip. Use slight force to engage metal clip.
4. Insert pushpin and secure into flanged hole.
To repeat, there is a black plastic pushpin retainer (
the round head is about 1" in diameter so you can grab it with your fingers) on the driver's side, middle of the outside edge of the tray, where it sits up against the horizontal sheetmetal behind the backseat bolster. It might be easier to see if you fold down the seat back, lay on your back and use a flashlite to help spot it, or climb into the trunk, whichever is easiest as long as you can look at the end of the tray towards the driver's side shock. Should be a simple process when you know where to look.
• See attached image below from the manual that shows the pushpin (12-32-18, accessing the ABS controller).
• Look at the photos of the tray, out of the car, above. The arrow on the left in comment #7:
http://www.sccoa.com/forums/showthread.php?13955-90-ABS-trunk-package-tray&p=74711#post74711 points to the flange on the tray where the retainer goes thru.
• Look also at the top of the photo of the tray that I added.
Grab the head of the pin and pull it down slightly until it unlocks, then pull down on the tray just enough to unseat the slotted metal clip, just be ready to hold the tray in case it wants to drop on that end and smack you in the head...might be the wiring harnesses will keep that from happening, just be aware. At this point you can lower that end of the tray and gain access to the wiring, relays, modules and JBL amp. The other end of the tray is hinged and won't come down as part of this process.
Before I figured out that one end of the tray would come down, I seem to recall just pushing up on the amp until it snapped out of the plastic retainers (
without dropping the tray), then fishing it down and out thru the large hole underneath it on that end of the tray, removing the connectors as needed to gain access and get it out.
If you insist on removing the -entire tray-, you also have to remove the two bolts that hold the tray hinge on the pass. side, from the top, inside the car, by lifting the material over the sheetmetal behind the bolster - don't forget the contents of the tray connect to wiring harnesses and you won't just be able to take the tray and contents straight from the trunk. Not something I recommend casually.
As for wanting to troubleshoot the components, AFAIK, failures to the factory JBL system (
throughly discussed here in the past) routinely involve either the head unit or the CD player, board level, so unless you have the skills to do those kinds of electronics repairs, and you've already gone thru both the head unit and the CD player, outside of all this being a good learning experience, you may be wasting your time checking the amps. I think the amps won't power up if the head unit is dead, but I'd still check fuses and wiring. If you decide you want to buy another/either factory JBL amp, place an ad in the want-to-buy section and I'll check my inventory.
If you do have board level electronic repair skills, I suspect there are others that will pay you to repair their JBL head units/CD players.