How to get new LED indicator cap for Rally RS200?

The pedal is not repairable.

You will need to contact Garmin support and see what they might do for you.

Is this this drive side pedal (right)?

For future installs on the drive side you will need to check the chain clearance and add one of the supplied washers if the gap is not large enough to prevent damage from the chain occurring. 

just thought i would make a comment here, as it seems to be a ridiculous state of affairs.

contacted Garmin support about exact same problem.  "We dont have a part number for that - send it in for a service".  They wouldnt even tell me what the part is called, they were more interested in getting me to send the pedals back to them for a fee.  So, having paid 1,000euros for RK200, I now have to pay for them to be serviced (as well as return freight costs), all for the sake of a small plastic part. 

Garmin, in terms of your product after-care / service capacity, you are a disgrace. 

The pedal would need to be exchanged for this. It is not a user-replaceable part. Please send me a direct message if you need further assistance.