This is possibly a very simple problem, caused either by the sensor (or switch) that detects the brakes have been applied, or a blown fuse that protects this electrical circuit. You could also have a broken wire somewhere within the loom, or even a problem with the body computer that controls many of a modern car’s functions. But start with the simple stuff first.

If the problem is an intermittent one, we’d definitely start looking at the switch that links the brake pedal to the brake lights.

Any excess smoke from a diesel engine can mean a too-rich mixture. So are you certain the injectors are the correct ones for that engine? Has a sensor been bumped during the replacement process that is not sending the right signal to the computer that controls the injection on a modern turbo-diesel?

Also, cold-start problems in an engine like this are often traced to the glow-plugs. These are crucial to warming up the cold engine internals so that combustion will take place. So perhaps a check of the glow-plug wiring is in order as this could have been disturbed while the injectors were being replaced.

Has anything else been changed or replaced as part of this process? Has the engine been sitting for an extended time (allowing the fuel to go stale)?

This is not an uncommon problem with the head unit in a lot of makes and models. Like any other computer-driven piece of equipment, they have a finite lifespan and, once they’re done, they’re usually done for good. That’s why the dealer would be recommending to replace the unit rather than tyring to fix the one you have.

What a lot of people do is that this opportunity to upgrade to a newer, better head unit with better sound, better connectivity and more up to date apps. There’s loads of choice in the aftermarket and you can spend as little or as much as you want. Many owners also use this as a chance to upgrade speakers and perhaps add a sub-woofer or extra amplifier for much better sound. And you’re right; if the original unit died, a replacement of the same type may well have the same problems.