If the appliance store is doing a healthy business and you have a good deal going there, that seems good to me. I'd see if you can try to differentiate your work for the appliance store as something you take in to your business (e.g. outside tech working on logic boards, etc.) and try to avoid having to wear an appliance store uniform/shirt - get a few of your own business shirts embroidered instead in a different color. Basically you want to complement and fit in with the appliance store because you want their customers to also become your customers, and you also want the appliance store folks happy to have you there.
I don't like your banner - it looks temporary but more important it's too small to read easily. Can you get an actual sign of your own, or at least a bigger banner? Put the contact information and other stuff on a door sign, not on the banner that's 12-15 feet up in the air - use the banner to suck them in ("Computer Repair" or "Computer & Electronics Repair"), then let them come to the door for more information. Getting a more permanent sign might also be good, but that'll require both permission from the store (ask them first) and probably city or zoning approval for signage - that might be harder without a separate physical address, so don't open that can of worms at all if the store says no.
Also, can you rearrange your shop so it's at least marginally customer-friendly? It doesn't have to have a browsing area, counter, anything like that, but it needs to be clean enough for people to bring PCs directly to you and feel comfortable when they drop them off. Also, if the floor is bare cement (not uncommon with a garage door like that) and you're going to do any rearranging, get one of those epoxy floor coverings applied. I've spent some time at a working glass studio that did this recently, and between a coat of paint, some shelves, and the non-cement floor it's like a completely different place and enormously more welcoming to customers.