So, I managed to find the sneak peak 1 on bing that I could watch.
https://www.bing.com/videos/search?q=The+Amazing+Race+-+Who+Has+This+One+in+the+Bag+(Sneak+Peek+2)&docid=20691119476717&mid=C83CA4AF8FD034F1E196C83CA4AF8FD034F1E196&view=detail&FORM=VIREThe car park exit the teams came out of is on Oswald Street. See:
https://goo.gl/maps/x4hjUbB9MNCBV6tW8 You can access that carpark directly from inside of the train station, so they will have exited platform 2, (where they're seen running along platform in the preview) and then entered platform 15 to get to the car park. I'm guessing there will have been a clue box somewhere in the train station concourse, they could have then asked people in the train station how to get to Luss before finding their car.
In the previews/sneak peaks, you see a good few of the cars exiting the car park on to Oswald street towards argyle street, and then along argyle street (
https://goo.gl/maps/uYv2D2r6ctkDebFj9) towards the M8 turnoff. (The M8 is a major motorway which runs right through the city going to other major cities etc) Which would be the correct thing to do to go out towards Luss.
The Love Island Couple look to be on the m8 clydeside express way in the sneak peek (at the 'i just want to stay safe' clip) - which would kind of make sense for going to Oran Mor from town but not the most direct route, so I think this will be following the singing detour. (By the way - the accordion player was playing 'donald wheres your trousers' so I wonder if that's the tune they have to learn), but they could have taken a wrong turn and got on to here when trying to get on to the M8 to go to Luss.
You see the Holderness' also finding signs for the M8, and you see them passing the famous Lauriston Pub which would make sense after exiting either of the detours. Again, from where they were they could find the M8 to get them to Oran Mor.
You see them cross this bridge on Great Western Road near Oran Mor -
https://goo.gl/maps/PkDPX3evddaU9yhE9 - just next to Kelvinbridge tube station. But its weird because when they find Oran Mor accidently, they're coming from the opposite direction from where they've crossed the bridge (this is where she says "holy crap!")
https://goo.gl/maps/RA4euVEy2p3rmEgUA , so I wonder if they've overshot and turned around.