Heater issue + E30 - 1990

Hello, My 1990 E30 6cyl. has a problem. The heater guage goes to the top at the hot position when driving for awhile. The interior has a anti-freeze smell,
and there is no heat. After a bit, the temp guage went down, and the heat worked but the anti-freeze smell stays. Is it the heater core or thermostat? What is involved in the replacement of the heater core if that is the issue? I thank you for all replies.
Peter J. Alicea
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