If 85/86 are the coil, 30 is the common, 87 is N/O and 87a is N/C, the following should work.
The steady +ve marker circuit connects to 87a.
The marker lights connect to 30.
The switched +ve blinker wire connects to 87.
The switched +ve blinker wire connects to a circuit that turns on the relay for a time longer than the off-time of the blinker.
This configuration directs the blinker signal to the marker lamps while the blinker is flashing, and otherwise directs the marker signal to the marker lamps if the blinker is not on.
The time delay circuit can use a 555 timer, or simply a diode + capacitor + resistors + transistor, or just a diode and very large capacitor. Sorry, I'd draw a circuit, but don't have a means to get it on the computer at the moment.