W140 SRS light on after battery drain

[1998S280]

Hello all,
I only use my W140 once a month for a 300 mile round trip to my work office and when I moved at the weekend to give it a wash before this months journey the battery was completely flat. I tried a power pack and also tried jumping from a spare battery but neither worked so I put the battery on charge.
The battery was new in 2023 so I dont expect it to be the battery but did notice that when recharging it never reached "full". After 3 or 4 hours I gave it a go and it started OK but with the SRS light on.
I went to move the car yesterday to go for fuel as I intended to use the car tomorrow for the work journey but the battery was flat again, luckily I have a spare identical battery so I swapped them over and it started up but again the SRS light is and and doesnt go away even after 5 minutes of running.
I suspect a parasitic drain on the battery from somewhere causing the flat battery, any thoughts on this and the SRS light please?
ps, there isnt an OBD2 port but there is the 38 pin port, can codes be DIY erased via the 38 pin port?

thank you

 

[toby1]

Have you turned the steering lock-to-lock twice?

 

[1998S280]

Yes, it didnt have any effect

 

[alexanderfoti]

SRS module may have failed and may be causing your drain.
You can read the codes in the SRS module using an Icarsoft with a 38pin to OBD adapter (AFAIK). Although we use the C4 mux with HHTwin, I'm sure I have used an Icarsoft a while ago with an adapter.