It's the relay under the fuses in the fuse box in the engine compartment. I had this same thing happen and took 3 yrs to figure out. Only one side of the relay is making contact that is y only one side of lights flash. I took it out used a little bit of 1000 grit sandpaper to clean it a bit and also slightly bend the contact. It's been working perfectly for the last few years.

Search the forum for something like siren with lock unlock. It will show the relay in question.

I'm on my phone now or I would give a link.