When mortgage down, bond yield usually also down and the bond price goes up. When bond price in the uptrend, investor usually chase bond and abandon stock market. Bond market is much more bigger than stock market.
There is also some research from ECRI (ECONOMIC CYCLE RESEARCH INSTITUTE). However, they do not use mortgage rate as indicator.
Typically, when bond rates (also known as the bond yield) go up, interest rates go up as well. And vice versa. Don’t confuse this with bond prices, which have an inverse relationship with interest rates.