"The external system is not connected"!?

[GEN]

On my 2005 Ls430 the screen goes totally dark then displays "External not connected and Check system" on a light background. While the screen is either dark or light with "System check" displayed all the functions usually available through the screen are obviously not accessible although all functioning.  However when reverse is engaged the rear camera comes on and displays as usual!  Additionally the fault is not permanent as, from time to time, the screen resumes normal function and then fails and then comes back again (this was repeated during lengthy drives over a period of 3 days, so far).  It feels as though it's a wire that disconnects and then reconnects... But if so, where to look?  Any thoughts, suggestions would be appreciated. 

[BigBoomer]

Could be almost anywhere from behind the screen to the amp/nav units in the boot.
Tracing intermittent wiring faults like that are a PITA I'm sorry to say.

[GEN]

Thank you Big Boomer.  If/when I find out will let you all know.  The only electrical work recently done (by Lexus dealer) was to replace both side mirrors.  Having experienced it further it is not a loose wire (not related to movement but time since the car was started:  after 4-5 miles it will fail; let it cool off for 30-60 minutes it resets and then repeat as before). 

4 hours ago, BigBoomer said:

Could be almost anywhere from behind the screen to the amp/nav units in the boot.
Tracing intermittent wiring faults like that are a PITA I'm sorry to say.