For all of you out there still searching for the Infernus.
After you have done some of the Brucie missions you get the phone call from Stevie, who texts you pictures of cars he wants you to collect. One of these cars is the Infernus.
There are no GPS markers to help you find the cars and it's not strictly speaking a main mission.
If you haven't already delivered the car then collect it and drive straight to a safehouse.
The place to get it is on the 1st Island just above the roundabout that is on the top left of the bottom park (Outlook Park). The road itself is called 'Montauk Ave', it heads north from roundabout. (You may have to wait for Stevie to request it tho which is done randomly!)
If you want more than one of them (most people would!), then keep reading.
On your way to the safehouse you will see more Infernus's on the road. (If not keep driving around til you do!).
You need to get on the same road as safehouse, then drive up and dowm in the Infernus until you see another one close(ish) to your safehouse parking area.
The trick is to keep your car in range whilst you jump out and steal a second Infernus. (If you get too far away from 1st car it will disappear). If you manage to get the second car and can still see the first one then go back to it and try to push one of them towards your parking zone. Once inside the zone it's safe. (Either save right away or drive to another safehouse to save game). This way you get both cars saved. If cops see you steal 2nd car then you may have to try again when you've lost them!
Once the 1st car is saved you need to repeat the process again. Remembering to always take your car back to a second space outside a safehouse before saving.
It can be a lengthy process but I've now got an Infernus outside every safehouse. If I crash one I just repeat the process again later on. As long as you have at least one saved then you can have unlimited Infernus's.
Obviously, this process can be used for all cars that you want to keep, unless your on a mission that involves it directly.
Anyway, hope this helps you. I'd been looking for them for ages and couldn't find any help anywhere!.