Changed rear tail light lamps to LED type

Electronic flashers are the expensive answer to a problem that never existed. Your new lights don't draw enough current to be detected by the flasher. You can add in a resistor across each bulb to increase the amount of current flow. As a guess, and a starting point, a typical brake light bulb draws an amp of current, so using Ohm' Law, that means it has about 12 ohms of resistance. Your new bulbs have WAY more than that. I'd start with about a 15 to 20 ohm resistor. Doing the math again, that would need to be about a 15 watt resistor. You can find something close in old tvs. You can also splice in additional sockets to put inside the trunk, then just use the original bulbs.