Guy's like a lot here I've got a TL with too many k's on the clock and I'm now checking the fuel and topping up the oil. But that's a story for another time.

The horn has failed. The circuit is sound. The two after market horns I bought either died or were so quiet they failed to do their job of making a warning noise.

I've since got a couple of horns that are quite good. I even managed to set up a relay and all was well other than the jerry rigged 12V line from the battery across the firewall and down the drivers side of the engine bay.

Is there a constant 12V rail(doesn't require the ignition on) I can tap on the driver side of the engine bay? The horn is also the alarm and while the alarm works, without a permanent 12V supply the horn doesn't actually sound.(the permanent 12V is for the relay not the horn switch itself)

The horn is already relayed (at least on the TJ, I wouldn't think the TL would be different in this respect) so unless your replacement horns draw significantly more current than the standard horn (fused at 10A) you probably don't really need to wire up another one. Not sure re +12V battery voltage availability near the standard horn position, your safest bet is probably to do what you did, running a wire of suitable thickness and with an in-line fuse directly from the battery.