What happens? Does the engine turn over but it won't start at all? Does it do nothing when you turn the key? Will it start and run for only a second or two and die? The battery going dead all the time? If the battery is going dead there may be a short, something keeping the car "awake" and draining the battery, or the charging system not working. Some new euro cars can't have the key left in it overnight(like if you park in a garage) without messing up the antitheft or draining the battery. Don't know if a Toureg is that way too or not.

The early (2004) Touaregs had many problems with no-starts, especially high-option models with navigation and all the goodies. (The CarTalk guys had one die when they were first reviewing the model.) VW did a campaign to update the older models (firmware flash, harness upgrades, etc.) about a year ago - was this work done (free)?
Another issue has been battery cables making poor contact and causing no-start. I believe there was a TSB on that as well.
An aftermarket alarm could be suspect, as tapping into the fancy wiring system can certainly cause problems. The TReg normally watches battery voltage and disconnects circuits if they drain it too low, but if you tie in another device it may not be so protected.