Matt's suggestion with AC based menu triggering would work fine, though if you stop the car and that turns off charging, you might get an extra popup.
Here's how you create it:
Replace the context for the profile you already have with Event->Variable->Variable Set: Variable: %Carmode, Value: on.
Create a new profile, "Carmode on". Contexts should be the following two:
Application:the car app you use
State->Power->Power: Source: AC (or USB, depending on what the phone registers your car charger as. probably AC).
Task should be the following:
Variable->Variable Set: Name: %Carmode, To: on
Create another new profile, "Carmode off". Contexts should be the following two:
State->Power->Power: Source: Any, Invert: checked
State->Variable->Variable value: Name: %Carmode, Op: Matches, Value: on
Task should be the following:
Variable->Variable Clear: Name: %Carmode
This will cause the following to happen:
The menu will appear when the phone is plugged into charging AND the car app is started.
It won't appear again until the phone has been unplugged, and the above two conditions are true once again.
Like I said though, if it stops charging and then starts back up again, e.g. because yous topped the car for a bit, the menu will pop back up. If that's a potential problem, my original suggestion might be the better choice