I got 3 new garden lights - the ones with 1 white LED in them that charge from a solar panel

1 of them does not work. I opened and 1 of the wires from the solar panel is disconnected - the drop of soldering is on the wire but it detached from the soldering spot on the back of the solar panel

The panel is a bit of glass with everything etched on its back

The soldering spot is metllic, i touch the wire to it and everything works ok. But it just won't take the soldering - It does not stick to it no matter what i try (a lot of solder, hold for a while, scratch it....)

The other wire is plain soldered to the spot - There is no issue of a metal tab that broke off and such

How to solder to it ?

I ran in to the same problem as you did on solder not sticking. but lead solder helps
 I endended up using hot melt glue over the wire on the thin metal on the back of the solar glass and it worked out just fine. or try to find conductive epoxy.

From what I've seen in stuff about building DIY PV modules, flux is needed.  The metal contacts on the back are probably some kind of aluminum alloy, so that is why they are hard to solder.

The easiest way to deal with this is to use a conductive liquid silver pen (can get it at radio shack).  After putting a drop of this conductive liquid and letting it fully dry for at least 4 hours or so you'll be able to solder to it.

Jeff

I spot somewhere a notice, than these require some special alloy, the regular tin-lead, nor unleaded won't work.
There are two aspects: The solder alloy should not dissolve the thin metalized film (regular Pb-Sn does) and it should melt on low enough temperature, so the soldering heat does not cause the glass to crack.

I think the conductive glue would be the best - you do not have to heat up the joint (so no risk of the glass to crack), you won't dissolve the thin metal layer into any solder and yet it would firmly hold the joint.

The solder there does melt at a very low temperature. Go to a auto spares supplier and get a pack of demister fixer, which is a silver loaded glue that will stick to the glass and the copper wire. Expensive, so try first a small piece of copper shim ( strip off a old PC board and clean both sides to a shine) and solder the wire to it, then place over the contact and epoxy it in place, so that it is held in contact with the panel. Cheapest and most likely to work, or else you will have to bin it.