Hhola, hahaha, sweat the fat drop, I am with a chub in a sling, and another one already came, which a priori is not easy, I am not electronic, I am a technician and mechanical engineer, out of necessity I have had to self -considerate in the issues of electronics basically because almost all the new cars are like this citroen ... personally I venture up to relay and signs of activation, I have electronic "pure" chips, memoirs, programming, etc.
Returning to Citroen, about the need to amplify the signal that you take from the telecoming, yes, your appreciation is correct, it is most likely ...
now in the BSI, for me the most likely, it is that you reach 2 relays because, depending on the type of Bsi and origin, use 1 reliable for the signaling and the other for the hazard or arrest lights ... It would be plans, examine the behavior of the circuit of the side lights that if they work, and if the second relay is for hazard, then having clear the operation within the BSI of the OK circuit, the bad tested ... I do not know if you understand me, there are times that I complicate myself in excess ... when explaining something simple .... hahaha. Finally if you turn on the hazard, both sides work? When do you give to the bad side, the module of the Istrums board should issue signaling sound signal? This happens like this or not? I test live circuits inside computers, such as this BSI, disarming them, isolating them with a cloth (care with the short) and I measure what I want to verify, obviously with the connectors put, only with the BSI "piluchita" hahaha, like almost all ... "they" look better devoid of their clothes ... hahaha. Greetings friend, I am on the lookout.