w169 starts but not with key

[Tombob]

My partner's W169 A170 auto won't start. The key turns all the way but starter doesn't turn (nor is there a solenoid "click"). The roadside mechanic speculated immobiliser.
It was towed to mechanic friend who thought starter motor but he started and ran the car by turning the key to "on" and powering the solenoid directly ... I guess this rules out the immobiliser.
I don't know if it's coincidence but the display also showed "Display faulty Visit workshop" and "SRS Restraint system faulty Drive to workshop" at this same time.
My mechanic friend also found error codes relating to slow starter motor, which made him suspect the starter (or solenoid).

Any thoughts? Relay (which one and where)?

T

 

[Blobcat]

I’d be checking battery for condition and leads for any black corrosion

 

[Tombob]

Thanks Blobcat.
Battery tested OK and no corrosion on the battery leads or wiring to the solenoid. I'm still thinking relay ... somewhere ...

 

[Tombob]

(bump?)
Mechanic just send a message saying it now starts and was ready for pickup ... but quickly another saying it now won't start.
My mechanics have worked on these cars over the years but are not Benz specialists. I'm hoping someone can offer some clues as the W169 was much more popular in the UK (I'm in Sydney ... where your ladies have just beaten our ladies in the WWC. Well played).

 

[Tombob]

(Expensive) update.
My mechanic's auto electrician spent an entire day on the car. I understand he swapped out all relays (except the fuel pump relay I'd swapped out a few months ago) until the car started with the key. It doesn't sound very scientific but at least it starts again.
AUD $1500 bill ...