I designed and manufacture LED taillights for the BMW 2002 -
www.2002s.net . Due to the circuitry design, ours maintain the oem flash rate.
The issue the OP describes: Flash rate too high, is the result of not enough resistance in the circuit.
The workaround is to replace the oem bi-metal electro-mechanical flasher with an electronic flasher unit which doesn't rely on electrical resistance to establish the flash rate.
These can be had from any NAPA, PepBoys, Autozone, etc. usually for under $10.
Problem solved!
Cheers!