[Noriyasu]

Hi,

If not just the fuel pump but all the lights don't come on then it could be an electrical issue. It may not be that serious.

First, I attached a photo around battery and resistor of my SVX. I can not find any black plastic electronic box there. If you're talking about the unit under the main fuse box, it's the cruise control actuator. I don't think you need to think about cruise control for now.

Have you checked the box fuses called SBFs as well as the blade fuses?
They are four red ones and two green ones in the picture.

for example, if the room lamp (I think you call it the dome lamp) doesn't come on when you open the door, it it related to the No.15 blade fuse in the sub fuse box under the A-pillar (under the driver's seat side) and SBF5 in the main fuse box.

The interior lamps are controlled by a delay unit for delay in turning off. It's a little complicated.

To make matters simpler, check your trunk lighting. It comes on by opening the trunk without turning the key as long as the battery has power.
Only the battery and FL1.5 and SBF3 and the blade fuse FB25 (10A) are related to the lighting in the trunk.

No ECU is involved in trunk lighting. So, if the trunk lights come on, some battery power line is available.

Maybe the black box you found is an immobilizer added by someone? Did you buy the `92SVX new? Didn't you add an optional immobilizer?
If you remember having the immobilizer on, I recommend bypassing it.

wish you could take a picture of it and upload it.