I am reporting Successful import of 2019 GLC 300, in November, 2023 which did not have TPMS. I received a lot of information from this thread, so I think it is my duty to report back so that others can benefit.
I bought TPMS from Amazon and got it installed by Townfair tire in Woonsocket Massachussets. He only charged me $80 for wheel alignment.
I filled up the forms and put the receipt of TPMS in a folder and drove to USA Cornwal Border in Ontario. The address of USBP port is as follows:
30M Seaway International Bridge
Rooseveltown, NY 13683
United States
The border agent was super friendly and very knowledgeable. She looked at the documents and said, you know what, they will make you fix the TPMS. I said I have done that. She checked the TPMS receipt and said, you are all set. She instructed me to park the car in designated spot and enter the office. She instructed her junior to do the work and told that all documents were in order. The office did not ask a single question and neither inspected the car.
I recommend this border crossing for importing the car with third party TPMS. Your experience may vary depending on which crossing you go to.
Staff here is very friendly. When we were waiting for our paperwork to be processed another person came for enquiry to import the car and border agent was explaining all details to her patiently. Even giving contact number of OEM to get compliance letter.
I had struggle with Mercedes Benz for getting the compliance letter. They said that as per policy they no longer issue letters stating that car meets all requirements except FMVSS 138. I tried escalating to CEO. There was still no response. I sued Mercedes in small claims court of Brampton. The process is online. This forced Mercedes to respond and opened a communication channel with their para legal, who was responding to my claim. I said that I will drop the claim if they gave letter stating that the car meets all requirements except FMVSS 138 (TPMS). They gave the letter.
Today on 11/30 I have been able to register my car in Massachussets. I hope this report helps someone.